[PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Andre Przywara]

Hi,

a normal Linux kernel currently supports reading the start and end 
address of a single binary blob via the FDT's /chosen node.
This will be interpreted as the location of an initial RAM disk.

The Xen hypervisor itself is a kernel, but needs up to _two_ binaries 
for proper operation: a Dom0 Linux kernel and it's associated initrd.
On x86 this is solved via the multiboot protocol used by the Grub 
bootloader, which supports to pass an arbitrary number of binary modules 
to any kernel.

Since in the ARM world we have the versatile device tree, we don't need 
to implement the mulitboot protocol.

So I'd like to propose a new binding which denotes binary modules a 
kernel can use at it's own discretion.
The need is triggered by the Xen hypervisor (which already uses a very 
similar scheme), but the approach is deliberately chosen to be as 
generic as possible to allow future uses (like passing firmware blobs 
for devices or the like).
Credits for this go to Ian Campbell, who started something very similar 
[1] for the Xen hypervisor. The intention of this proposal is to make 
this generic and publicly documented.

Looking forward to any comments!

Thanks,
Andre.

[1] 
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/arm/device-tree/booting.txt;h=94cd3f18a4e1317a35e1255bf5c6e1e091001d1a;hb=HEAD
----------------------------
* Multiple boot modules device tree bindings

Boot loaders wanting to pass multiple additional binaries to a kernel 
shall add a node "module" for each binary blob under the /chosen node 
with the following properties:

- compatible:
     compatible = "boot,module";
   A bootloader may add names to more specifically describe the module,
   e.g. Xen may use "xen,dom0-kernel" or "xen,dom0-ramdisk".
   If possible a kernel should be able to use modules even without a
   descriptive naming, by enumerating them in order and using hard-coded
   meanings for each module (e.g. first is kernel, second is initrd).

- reg: specifies the base physical address and size of a region in
   memory where the bootloader loaded the respective binary data to.

- bootargs:
   An optional property describing arguments to use for this module.
   Could be a command line or configuration data.

Example:
/chosen {
     #size-cells = <0x1>;
     #address-cells = <0x1>;
     module@0 {
         compatible = "xen,linux-zimage", "xen,multiboot-module", 
"boot,module";
         reg = <0x80000000 0x003dcff8>;
         bootargs = "console=hvc0 earlyprintk ro root=/dev/sda1 nosmp";
     };
     module@1 {
         compatible = "xen,linux-initrd", "xen,multiboot-module", 
"boot,module";
         reg = <0x08000000 0x00123456>;
     };
...

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Rob Herring]

On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara
<andre.przywara-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> Hi,
>
> a normal Linux kernel currently supports reading the start and end address
> of a single binary blob via the FDT's /chosen node.
> This will be interpreted as the location of an initial RAM disk.
>
> The Xen hypervisor itself is a kernel, but needs up to _two_ binaries for
> proper operation: a Dom0 Linux kernel and it's associated initrd.
> On x86 this is solved via the multiboot protocol used by the Grub
> bootloader, which supports to pass an arbitrary number of binary modules to
> any kernel.
>
> Since in the ARM world we have the versatile device tree, we don't need to
> implement the mulitboot protocol.

But surely there would be some advantage of reuse by using the
multi-boot protocol since Xen, grub, and OS tools already support it
for x86.

> So I'd like to propose a new binding which denotes binary modules a kernel
> can use at it's own discretion.
> The need is triggered by the Xen hypervisor (which already uses a very
> similar scheme), but the approach is deliberately chosen to be as generic as
> possible to allow future uses (like passing firmware blobs for devices or
> the like).
> Credits for this go to Ian Campbell, who started something very similar [1]
> for the Xen hypervisor. The intention of this proposal is to make this
> generic and publicly documented.

Can you describe how you see the boot flow working starting with OS
installer writes kernel, initrd, xen and ??? to disk. How does the
bootloader know what to load? The OS may not have access to the dtb,
so this has to be described to the bootloader as well.

>
> Looking forward to any comments!
>
> Thanks,
> Andre.
>
> [1]
> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/arm/device-tree/booting.txt;h=94cd3f18a4e1317a35e1255bf5c6e1e091001d1a;hb=HEAD
> ----------------------------
> * Multiple boot modules device tree bindings
>
> Boot loaders wanting to pass multiple additional binaries to a kernel shall
> add a node "module" for each binary blob under the /chosen node with the
> following properties:
>
> - compatible:
>     compatible = "boot,module";
>   A bootloader may add names to more specifically describe the module,
>   e.g. Xen may use "xen,dom0-kernel" or "xen,dom0-ramdisk".
>   If possible a kernel should be able to use modules even without a
>   descriptive naming, by enumerating them in order and using hard-coded
>   meanings for each module (e.g. first is kernel, second is initrd).
>
> - reg: specifies the base physical address and size of a region in
>   memory where the bootloader loaded the respective binary data to.
>
> - bootargs:
>   An optional property describing arguments to use for this module.
>   Could be a command line or configuration data.

> Example:
> /chosen {
>     #size-cells = <0x1>;
>     #address-cells = <0x1>;
>     module@0 {
>         compatible = "xen,linux-zimage", "xen,multiboot-module",
> "boot,module";
>         reg = <0x80000000 0x003dcff8>;
>         bootargs = "console=hvc0 earlyprintk ro root=/dev/sda1 nosmp";
>     };
>     module@1 {
>         compatible = "xen,linux-initrd", "xen,multiboot-module",
> "boot,module";
>         reg = <0x08000000 0x00123456>;
>     };

This has to be created and parsed typically in FDT format by early
boot code, and I worry about the complexity this has. Being future
proof and extensible is good, but we could meet today's needs with
something simple like this:

bootargs = "xen args --- linux args";
xen,linux-image = <start size>;

So, is having a more generic solution really needed?

Rob

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Andre Przywara]

On 09/04/2013 12:00 AM, Rob Herring wrote:
> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara

Hi Rob,

>> a normal Linux kernel currently supports reading the start and end address
>> of a single binary blob via the FDT's /chosen node.
>> This will be interpreted as the location of an initial RAM disk.
>>
>> The Xen hypervisor itself is a kernel, but needs up to _two_ binaries for
>> proper operation: a Dom0 Linux kernel and it's associated initrd.
>> On x86 this is solved via the multiboot protocol used by the Grub
>> bootloader, which supports to pass an arbitrary number of binary modules to
>> any kernel.
>>
>> Since in the ARM world we have the versatile device tree, we don't need to
>> implement the mulitboot protocol.
>
> But surely there would be some advantage of reuse by using the
> multi-boot protocol since Xen, grub, and OS tools already support it
> for x86.

Yes, but that is x86 only and multiboot is it's nature quite 
architecture specific. The current(?) multiboot v2 spec has no official 
ARM support (only x86 and MIPS), so this would need to be "invented" 
first. While this is technically easy, ARM software currently has no 
support for multiboot at all: not in u-boot and not in Xen.
Multiboot support in Xen lives entirely in the x86 directory, and big 
parts of it are even in assembly.

I am about to write up a more elaborate technical rationale describing 
the problems with multiboot on ARM:

https://wiki.linaro.org/AndrePrzywara/Multiboot

>> So I'd like to propose a new binding which denotes binary modules a kernel
>> can use at it's own discretion.
>> The need is triggered by the Xen hypervisor (which already uses a very
>> similar scheme), but the approach is deliberately chosen to be as generic as
>> possible to allow future uses (like passing firmware blobs for devices or
>> the like).
>> Credits for this go to Ian Campbell, who started something very similar [1]
>> for the Xen hypervisor. The intention of this proposal is to make this
>> generic and publicly documented.
>
> Can you describe how you see the boot flow working starting with OS
> installer writes kernel, initrd, xen and ??? to disk. How does the
> bootloader know what to load? The OS may not have access to the dtb,
> so this has to be described to the bootloader as well.

The idea is to use bootscripts (for instance in u-boot) to tackle this. 
See for an example below.

I don't see how the process would be differ significantly from the 
current process, where you have to load mostly two images, get hold of 
the DTB, enter image data into the DTB and launch the kernel. Now you 
just need to load an additional image and enter it's properties into the 
DTB, actually in a pretty generic way.

>>
>> Looking forward to any comments!
>>
>> Thanks,
>> Andre.
>>
>> [1]
>> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/arm/device-tree/booting.txt;h=94cd3f18a4e1317a35e1255bf5c6e1e091001d1a;hb=HEAD
>> ----------------------------
>> * Multiple boot modules device tree bindings
>>
>> Boot loaders wanting to pass multiple additional binaries to a kernel shall
>> add a node "module" for each binary blob under the /chosen node with the
>> following properties:
>>
>> - compatible:
>>      compatible = "boot,module";
>>    A bootloader may add names to more specifically describe the module,
>>    e.g. Xen may use "xen,dom0-kernel" or "xen,dom0-ramdisk".
>>    If possible a kernel should be able to use modules even without a
>>    descriptive naming, by enumerating them in order and using hard-coded
>>    meanings for each module (e.g. first is kernel, second is initrd).
>>
>> - reg: specifies the base physical address and size of a region in
>>    memory where the bootloader loaded the respective binary data to.
>>
>> - bootargs:
>>    An optional property describing arguments to use for this module.
>>    Could be a command line or configuration data.
>
>> Example:
>> /chosen {
>>      #size-cells = <0x1>;
>>      #address-cells = <0x1>;
>>      module@0 {
>>          compatible = "xen,linux-zimage", "xen,multiboot-module",
>> "boot,module";
>>          reg = <0x80000000 0x003dcff8>;
>>          bootargs = "console=hvc0 earlyprintk ro root=/dev/sda1 nosmp";
>>      };
>>      module@1 {
>>          compatible = "xen,linux-initrd", "xen,multiboot-module",
>> "boot,module";
>>          reg = <0x08000000 0x00123456>;
>>      };
>
> This has to be created and parsed typically in FDT format by early
> boot code, and I worry about the complexity this has. Being future
> proof and extensible is good, but we could meet today's needs with
> something simple like this:

Parsing this is already done in Xen, for instance. In fact we just look 
for nodes matching "boot,module" and then check for other names to 
determine it's type (which is a few-liner patch in Xen which I will post 
later today). And I don't see a need to load modules that early that we 
don't have an un-flattened tree available.

Generating is also part of libfdt, in fact this whole subtree above has 
been generated on the command line of a stock Calxeda U-Boot:
dom0kernel=fdt addr ${fdt_addr}; fdt resize; fdt mknod /chosen module@0; 
fdt set /chosen/module@0 compatible "xen,linux-zimage" 
"xen,multiboot-module" "boot,module"; fdt set /chosen/module@0 reg 
<${dom0_addr_r} 0x${filesize}>; fdt set /chosen/module@0 bootargs 
"console=hvc0 earlyprintk ro root=/dev/sda1 nosmp"

With this you load the Dom0 kernel (via TFTP or ext2load) and do "run 
dom0kernel" and are done. I have also patches which add an u-boot 
command called "module" which automates this, but this is mostly 
syntactic sugar (though may be useful for future abstraction, for 
instance to support the x86 (or even ARM) "real" multiboot protocol).

> bootargs = "xen args --- linux args";
> xen,linux-image = <start size>;
>
> So, is having a more generic solution really needed?

Not necessarily needed, but useful, I think. As described above I don't 
see any technical obstacles of doing it in a more generic way, so we 
could as well go ahead with this. On x86 from time to time the need for 
additional binaries pops up (early microcode loading, for instance), so 
why not be be prepared.
Also this approach avoids hard-coding the Xen name into the bootloader, 
as said in the proposal the meaning could be derived from the order of 
the modules (as on x86), so a bootloader does not need to know anything 
about Xen at all.

Regards,
Andre.

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Ian Campbell]

On Wed, 2013-09-04 at 10:43 +0200, Andre Przywara wrote:

> I am about to write up a more elaborate technical rationale describing 
> the problems with multiboot on ARM:
> 
> https://wiki.linaro.org/AndrePrzywara/Multiboot

Doesn't seem to exist? A search for "mulitboot" doesn't seem to throw up
the one you meant either.


> > So, is having a more generic solution really needed?
> 
> Not necessarily needed, but useful, I think. As described above I don't 
> see any technical obstacles of doing it in a more generic way, so we 
> could as well go ahead with this. On x86 from time to time the need for 
> additional binaries pops up (early microcode loading, for instance), so 
> why not be be prepared.

I agree. There have also been occasions where people doing
disaggregation have wanted to start multiple initial domains, requiring
additional modules at load time. I don't think being generic and
extensible is costing too much here.

Ian.

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Andre Przywara]

On 09/04/2013 10:55 AM, Ian Campbell wrote:
> On Wed, 2013-09-04 at 10:43 +0200, Andre Przywara wrote:
>
>> I am about to write up a more elaborate technical rationale describing
>> the problems with multiboot on ARM:
>>
>> https://wiki.linaro.org/AndrePrzywara/Multiboot
>
> Doesn't seem to exist? A search for "mulitboot" doesn't seem to throw up
> the one you meant either.

Try again now. As mentioned "I am about to write ..." ;-)

Thanks,
Andre.

>>> So, is having a more generic solution really needed?
>>
>> Not necessarily needed, but useful, I think. As described above I don't
>> see any technical obstacles of doing it in a more generic way, so we
>> could as well go ahead with this. On x86 from time to time the need for
>> additional binaries pops up (early microcode loading, for instance), so
>> why not be be prepared.
>
> I agree. There have also been occasions where people doing
> disaggregation have wanted to start multiple initial domains, requiring
> additional modules at load time. I don't think being generic and
> extensible is costing too much here.
>
> Ian.
>

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Rob Herring]

On Wed, Sep 4, 2013 at 3:43 AM, Andre Przywara
<andre.przywara-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> On 09/04/2013 12:00 AM, Rob Herring wrote:
>>
>> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara

>> But surely there would be some advantage of reuse by using the
>> multi-boot protocol since Xen, grub, and OS tools already support it
>> for x86.
>
>
> Yes, but that is x86 only and multiboot is it's nature quite architecture
> specific. The current(?) multiboot v2 spec has no official ARM support (only
> x86 and MIPS), so this would need to be "invented" first. While this is
> technically easy, ARM software currently has no support for multiboot at
> all: not in u-boot and not in Xen.
> Multiboot support in Xen lives entirely in the x86 directory, and big parts
> of it are even in assembly.

If so, I cannot find the assembly and most of it seems to be moved out
of x86 code now.

Rob

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Andre Przywara]

On 09/05/2013 03:23 PM, Rob Herring wrote:
> On Wed, Sep 4, 2013 at 3:43 AM, Andre Przywara
> <andre.przywara-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>> On 09/04/2013 12:00 AM, Rob Herring wrote:
>>>
>>> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara
>
>>> But surely there would be some advantage of reuse by using the
>>> multi-boot protocol since Xen, grub, and OS tools already support it
>>> for x86.
>>
>>
>> Yes, but that is x86 only and multiboot is it's nature quite architecture
>> specific. The current(?) multiboot v2 spec has no official ARM support (only
>> x86 and MIPS), so this would need to be "invented" first. While this is
>> technically easy, ARM software currently has no support for multiboot at
>> all: not in u-boot and not in Xen.
>> Multiboot support in Xen lives entirely in the x86 directory, and big parts
>> of it are even in assembly.
>
> If so, I cannot find the assembly and most of it seems to be moved out
> of x86 code now.

There is some code that relocates and maps the multiboot structure: in 
files in the xen/arch/x86/boot directory, including head.S, reloc.c and 
setup.c.
That would need to be completely rewritten, not only because it deals 
with multiboot v1, but also because it is scattered throughout the x86 
setup code and the initial page setup is different from ARM.

All these things are also done on ARM - but for the device tree already.
Of course proper multiboot v2 support is doable - but please compare the 
effort needed for this against this one:
http://lists.xen.org/archives/html/xen-devel/2013-09/msg00359.html

Regards,
Andre.

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Ian Campbell]

On Tue, 2013-09-03 at 17:00 -0500, Rob Herring wrote:
> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara
> <andre.przywara@linaro.org> wrote:
> > Hi,
> >
> > a normal Linux kernel currently supports reading the start and end address
> > of a single binary blob via the FDT's /chosen node.
> > This will be interpreted as the location of an initial RAM disk.
> >
> > The Xen hypervisor itself is a kernel, but needs up to _two_ binaries for
> > proper operation: a Dom0 Linux kernel and it's associated initrd.
> > On x86 this is solved via the multiboot protocol used by the Grub
> > bootloader, which supports to pass an arbitrary number of binary modules to
> > any kernel.
> >
> > Since in the ARM world we have the versatile device tree, we don't need to
> > implement the mulitboot protocol.
> 
> But surely there would be some advantage of reuse by using the
> multi-boot protocol since Xen, grub, and OS tools already support it
> for x86.

Multiboot is pretty x86 specific (although MB2 has a MIPS port) and
covers more stuff than we strictly require (e.g. on x86 it has
requirements around which processor mode you enter in, has paging
enabled etc).

> > So I'd like to propose a new binding which denotes binary modules a kernel
> > can use at it's own discretion.
> > The need is triggered by the Xen hypervisor (which already uses a very
> > similar scheme), but the approach is deliberately chosen to be as generic as
> > possible to allow future uses (like passing firmware blobs for devices or
> > the like).
> > Credits for this go to Ian Campbell, who started something very similar [1]
> > for the Xen hypervisor. The intention of this proposal is to make this
> > generic and publicly documented.
> 
> Can you describe how you see the boot flow working starting with OS
> installer writes kernel, initrd, xen and ??? to disk.

Kernel and initrd are written to /boot in the usual way (probably from
kernel.deb or whatever). Xen would also normally come from a distro
package (also in /boot).

> How does the bootloader know what to load?

It's in the bootloader config, e.g. boot.scr or grub.cfg, which are
either hand written or produced by the distros tooling.

grub on ARM could consume the same stanzas as are used by grub on x86 to
boot Xen (which are produced by update-grub):
        echo    'Loading Xen 4.1-amd64 ...'
        multiboot       /xen-4.1-amd64.gz placeholder  
        echo    'Loading Linux 3.10-2-amd64 ...'
        module  /vmlinuz-3.10-2-amd64 placeholder root=/dev/mapper/disks-root ro resume=/dev/mapper/disks-swap quiet
        echo    'Loading initial ramdisk ...'
        module  /initrd.img-3.10-2-amd64

Since there is no multiboot on ARM (and never will be) this is safe.

If multiboot ever does come to ARM it will necessarily be multiboot2
which uses a different keyword.

For u-boot Andre has proposed some syntactic sugar over the "fdt"
command to make boot.scr more trivial to use. We would of course need to
implement support for using it in the relevant distro tools (but they
tend to be very distro/machine specific already, e.g. Debian's
flash-kernel)

>  The OS may not have access to the dtb,
> so this has to be described to the bootloader as well.

Right. The proposal is not for the OS to frob the DTB to support this
protocol but to configure the bootloader to do it.

Ian.

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Rob Herring]

Adding Dennis for a distro perspective.

On Wed, Sep 4, 2013 at 3:44 AM, Ian Campbell <Ian.Campbell-Sxgqhf6Nn4DQT0dZR+AlfA@public.gmane.org> wrote:
> On Tue, 2013-09-03 at 17:00 -0500, Rob Herring wrote:
>> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara
>> <andre.przywara-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>> > Hi,
>> >
>> > a normal Linux kernel currently supports reading the start and end address
>> > of a single binary blob via the FDT's /chosen node.
>> > This will be interpreted as the location of an initial RAM disk.
>> >
>> > The Xen hypervisor itself is a kernel, but needs up to _two_ binaries for
>> > proper operation: a Dom0 Linux kernel and it's associated initrd.
>> > On x86 this is solved via the multiboot protocol used by the Grub
>> > bootloader, which supports to pass an arbitrary number of binary modules to
>> > any kernel.
>> >
>> > Since in the ARM world we have the versatile device tree, we don't need to
>> > implement the mulitboot protocol.
>>
>> But surely there would be some advantage of reuse by using the
>> multi-boot protocol since Xen, grub, and OS tools already support it
>> for x86.
>
> Multiboot is pretty x86 specific (although MB2 has a MIPS port) and
> covers more stuff than we strictly require (e.g. on x86 it has
> requirements around which processor mode you enter in, has paging
> enabled etc).
>
>> > So I'd like to propose a new binding which denotes binary modules a kernel
>> > can use at it's own discretion.
>> > The need is triggered by the Xen hypervisor (which already uses a very
>> > similar scheme), but the approach is deliberately chosen to be as generic as
>> > possible to allow future uses (like passing firmware blobs for devices or
>> > the like).
>> > Credits for this go to Ian Campbell, who started something very similar [1]
>> > for the Xen hypervisor. The intention of this proposal is to make this
>> > generic and publicly documented.
>>
>> Can you describe how you see the boot flow working starting with OS
>> installer writes kernel, initrd, xen and ??? to disk.
>
> Kernel and initrd are written to /boot in the usual way (probably from
> kernel.deb or whatever). Xen would also normally come from a distro
> package (also in /boot).
>
>> How does the bootloader know what to load?
>
> It's in the bootloader config, e.g. boot.scr or grub.cfg, which are
> either hand written or produced by the distros tooling.
>
> grub on ARM could consume the same stanzas as are used by grub on x86 to
> boot Xen (which are produced by update-grub):
>         echo    'Loading Xen 4.1-amd64 ...'
>         multiboot       /xen-4.1-amd64.gz placeholder
>         echo    'Loading Linux 3.10-2-amd64 ...'
>         module  /vmlinuz-3.10-2-amd64 placeholder root=/dev/mapper/disks-root ro resume=/dev/mapper/disks-swap quiet
>         echo    'Loading initial ramdisk ...'
>         module  /initrd.img-3.10-2-amd64
>
> Since there is no multiboot on ARM (and never will be) this is safe.
>
> If multiboot ever does come to ARM it will necessarily be multiboot2
> which uses a different keyword.

Right, this is just a text file with a list of binaries. It is not
really the multiboot spec. There is no reason for this part to be
different for grub on ARM. There is a big advantage to reusing the
distro side tooling. If there isn't really much reuse on the
bootloader side, then I'm fine with a different bootloader to Xen
interface. I would like to hear that from folks working on grub
though.

> For u-boot Andre has proposed some syntactic sugar over the "fdt"
> command to make boot.scr more trivial to use. We would of course need to
> implement support for using it in the relevant distro tools (but they
> tend to be very distro/machine specific already, e.g. Debian's
> flash-kernel)

And being machine specific is a PITA. flash-kernel is certainly not
something we want to expand on. There is not much love for boot.scr
either. There is work to address what are not really machine
differences, but largely vendor u-boot differences:

http://www.mail-archive.com/u-boot-0aAXYlwwYIKGBzrmiIFOJg@public.gmane.org/msg119025.html

One option for u-boot which already supports syslinux style menu files
is to adopt the syslinux multiboot parsing support:

http://www.syslinux.org/wiki/index.php/Doc/mboot


We need to back-up and consider what this looks like in the end for
all the pieces and get input from folks on grub, UEFI, and armv8. The
UEFI answer may be this is a grub problem. For armv8, this proposal
does match up well as the kernel boot interface for v8 is DT. Despite
some claims, ACPI will not completely replace DT because of this.

Rob

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Grant Likely]

On Wed, Sep 4, 2013 at 5:41 PM, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> For u-boot Andre has proposed some syntactic sugar over the "fdt"
>> command to make boot.scr more trivial to use. We would of course need to
>> implement support for using it in the relevant distro tools (but they
>> tend to be very distro/machine specific already, e.g. Debian's
>> flash-kernel)
>
> And being machine specific is a PITA. flash-kernel is certainly not
> something we want to expand on. There is not much love for boot.scr
> either. There is work to address what are not really machine
> differences, but largely vendor u-boot differences:
>
> http://www.mail-archive.com/u-boot-0aAXYlwwYIKGBzrmiIFOJg@public.gmane.org/msg119025.html
>
> One option for u-boot which already supports syslinux style menu files
> is to adopt the syslinux multiboot parsing support:
>
> http://www.syslinux.org/wiki/index.php/Doc/mboot

Even building it into U-Boot is problematic because it leaves older
machines out in the cold. Leif's port of Grub to U-Boot is far more
interesting since the distro can now be in control of the code that
loads the images and jumps into the kernel/hypervisor.

> We need to back-up and consider what this looks like in the end for
> all the pieces and get input from folks on grub, UEFI, and armv8. The
> UEFI answer may be this is a grub problem. For armv8, this proposal
> does match up well as the kernel boot interface for v8 is DT. Despite
> some claims, ACPI will not completely replace DT because of this.

Yes, for UEFI it is absolutely an OS loader problem. UEFI provides an
API and runtime environment. Grub is in general moving towards a boot
menu system and a tool for loading images. Actual booting however
should be done by a separate OS loader application. For Linux, this
will be an in-kernel UEFI Stub. For Xen I would recommend taking the
Linux EFI stub code and doing the same thing. There really isn't a
need for a multiboot spec when you can rely on a runtime execution
environment for setting things up exactly as you want them.

Multiboot only makes sense to me when you need to rely on firmware or
something else out of your control to load the images in a particular
way.

g.
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Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Ian Campbell]

On Fri, 2013-09-13 at 11:13 +0100, Grant Likely wrote:
> On Wed, Sep 4, 2013 at 5:41 PM, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >> For u-boot Andre has proposed some syntactic sugar over the "fdt"
> >> command to make boot.scr more trivial to use. We would of course need to
> >> implement support for using it in the relevant distro tools (but they
> >> tend to be very distro/machine specific already, e.g. Debian's
> >> flash-kernel)
> >
> > And being machine specific is a PITA. flash-kernel is certainly not
> > something we want to expand on. There is not much love for boot.scr
> > either. There is work to address what are not really machine
> > differences, but largely vendor u-boot differences:
> >
> > http://www.mail-archive.com/u-boot-0aAXYlwwYIKGBzrmiIFOJg@public.gmane.org/msg119025.html
> >
> > One option for u-boot which already supports syslinux style menu files
> > is to adopt the syslinux multiboot parsing support:
> >
> > http://www.syslinux.org/wiki/index.php/Doc/mboot
> 
> Even building it into U-Boot is problematic because it leaves older
> machines out in the cold. Leif's port of Grub to U-Boot is far more
> interesting since the distro can now be in control of the code that
> loads the images and jumps into the kernel/hypervisor.

AIUI this is not being developed any further?

> > We need to back-up and consider what this looks like in the end for
> > all the pieces and get input from folks on grub, UEFI, and armv8. The
> > UEFI answer may be this is a grub problem. For armv8, this proposal
> > does match up well as the kernel boot interface for v8 is DT. Despite
> > some claims, ACPI will not completely replace DT because of this.
> 
> Yes, for UEFI it is absolutely an OS loader problem. UEFI provides an
> API and runtime environment. Grub is in general moving towards a boot
> menu system and a tool for loading images. Actual booting however
> should be done by a separate OS loader application. For Linux, this
> will be an in-kernel UEFI Stub.

I'm not sure I follow your distinction between loading the images and
"actual booting". If grub loads the images and Linux stub does the
actual booting how does this stub locate the images which grub loaded?

Or are you saying the stub should load the initramfs itself? How does it
know where to find it? Having the kernel in one config file (grub's) and
the initramfs in another (the Linux UEFI stub's) seems likely to result
in things getting out of sync. Having Linux's stub parse the grub CFG is
even less likely to work well IMHO.

>  For Xen I would recommend taking the
> Linux EFI stub code and doing the same thing. There really isn't a
> need for a multiboot spec when you can rely on a runtime execution
> environment for setting things up exactly as you want them.

If this works for Linux on EFI then I see no reason it could work for
Xen on EFI (assuming my questions above are just a result of my
misunderstanding something)

But... Xen also wants to support non-server and therefore non-EFI
systems i.e. u-boot. We also want to support v7 where EFI is not a given
even for servers AIUI.

Given that I think it is a given that Xen will have some sort of
protocol along these lines, for use in these environments even if it
does the EFI stub thing on EFI systems. The question is shall we make it
more general and useful to others or just go our own way? I'd prefer to
do the former.

> Multiboot only makes sense to me when you need to rely on firmware or
> something else out of your control to load the images in a particular
> way.

I think in some cases do end up needing to rely on that though.

Ian.

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Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Grant Likely]

On Fri, 13 Sep 2013 12:22:13 +0100, Ian Campbell <Ian.Campbell-Sxgqhf6Nn4DQT0dZR+AlfA@public.gmane.org> wrote:
> On Fri, 2013-09-13 at 11:13 +0100, Grant Likely wrote:
> > On Wed, Sep 4, 2013 at 5:41 PM, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > >> For u-boot Andre has proposed some syntactic sugar over the "fdt"
> > >> command to make boot.scr more trivial to use. We would of course need to
> > >> implement support for using it in the relevant distro tools (but they
> > >> tend to be very distro/machine specific already, e.g. Debian's
> > >> flash-kernel)
> > >
> > > And being machine specific is a PITA. flash-kernel is certainly not
> > > something we want to expand on. There is not much love for boot.scr
> > > either. There is work to address what are not really machine
> > > differences, but largely vendor u-boot differences:
> > >
> > > http://www.mail-archive.com/u-boot-0aAXYlwwYIKGBzrmiIFOJg@public.gmane.org/msg119025.html
> > >
> > > One option for u-boot which already supports syslinux style menu files
> > > is to adopt the syslinux multiboot parsing support:
> > >
> > > http://www.syslinux.org/wiki/index.php/Doc/mboot
> > 
> > Even building it into U-Boot is problematic because it leaves older
> > machines out in the cold. Leif's port of Grub to U-Boot is far more
> > interesting since the distro can now be in control of the code that
> > loads the images and jumps into the kernel/hypervisor.
> 
> AIUI this is not being developed any further?

Last I checked the patches have been posted for merging, but it is
stalled on the Grub maintainer. I believe he wanted to fix a bug on the
raspberry pi before merging. Leif would know more. LEG isn't actively
working on it anymore since UEFI is the priority, but I wouldn't call it
abandoned.

> > > We need to back-up and consider what this looks like in the end for
> > > all the pieces and get input from folks on grub, UEFI, and armv8. The
> > > UEFI answer may be this is a grub problem. For armv8, this proposal
> > > does match up well as the kernel boot interface for v8 is DT. Despite
> > > some claims, ACPI will not completely replace DT because of this.
> > 
> > Yes, for UEFI it is absolutely an OS loader problem. UEFI provides an
> > API and runtime environment. Grub is in general moving towards a boot
> > menu system and a tool for loading images. Actual booting however
> > should be done by a separate OS loader application. For Linux, this
> > will be an in-kernel UEFI Stub.
> 
> I'm not sure I follow your distinction between loading the images and
> "actual booting". If grub loads the images and Linux stub does the
> actual booting how does this stub locate the images which grub loaded?

(Note: this isn't implemented yet, but is in progress) The DTB will need
to be passed via the UEFI system table. Initrd is passed by modifying
the dtb with linux,initrd-* properties in the device tree.

In the Xen case, I think the original proposal is conceptually sound.
I'll quibble about a couple of the details, but I'll address those with
a reply to the original proposal. It makes perfect sense to use the same
mechanism as the initrd properties to tell the kernel about additional
blobs.

> Or are you saying the stub should load the initramfs itself? How does it
> know where to find it? Having the kernel in one config file (grub's) and
> the initramfs in another (the Linux UEFI stub's) seems likely to result
> in things getting out of sync. Having Linux's stub parse the grub CFG is
> even less likely to work well IMHO.

The stub has the ability to load both the dtb and initrd itself by
adding "dtb=" and "initrd=" arguments, but it is currently limited to
loading images from the same filesystem that the kernel was loaded from.
GRUB (or gummiboot/refind/etc) is potentially more flexible.

> >  For Xen I would recommend taking the
> > Linux EFI stub code and doing the same thing. There really isn't a
> > need for a multiboot spec when you can rely on a runtime execution
> > environment for setting things up exactly as you want them.
> 
> If this works for Linux on EFI then I see no reason it could work for
> Xen on EFI (assuming my questions above are just a result of my
> misunderstanding something)
> 
> But... Xen also wants to support non-server and therefore non-EFI
> systems i.e. u-boot. We also want to support v7 where EFI is not a given
> even for servers AIUI.

I was only responding to the UEFI question, but that aside, I don't see
a problem with adding properties to the DT for telling Xen where they
are.

> Given that I think it is a given that Xen will have some sort of
> protocol along these lines, for use in these environments even if it
> does the EFI stub thing on EFI systems. The question is shall we make it
> more general and useful to others or just go our own way? I'd prefer to
> do the former.

Yes, make it general. GRUB and the EFI stub will use it.

g.

Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Rob Herring]

On Fri, Sep 13, 2013 at 5:13 AM, Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org> wrote:
> On Wed, Sep 4, 2013 at 5:41 PM, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>> For u-boot Andre has proposed some syntactic sugar over the "fdt"
>>> command to make boot.scr more trivial to use. We would of course need to
>>> implement support for using it in the relevant distro tools (but they
>>> tend to be very distro/machine specific already, e.g. Debian's
>>> flash-kernel)
>>
>> And being machine specific is a PITA. flash-kernel is certainly not
>> something we want to expand on. There is not much love for boot.scr
>> either. There is work to address what are not really machine
>> differences, but largely vendor u-boot differences:
>>
>> http://www.mail-archive.com/u-boot-0aAXYlwwYIKGBzrmiIFOJg@public.gmane.org/msg119025.html
>>
>> One option for u-boot which already supports syslinux style menu files
>> is to adopt the syslinux multiboot parsing support:
>>
>> http://www.syslinux.org/wiki/index.php/Doc/mboot
>
> Even building it into U-Boot is problematic because it leaves older
> machines out in the cold. Leif's port of Grub to U-Boot is far more
> interesting since the distro can now be in control of the code that
> loads the images and jumps into the kernel/hypervisor.

Considering there is no distro support for grub on ARM yet, it may be
more interesting in the long run, but it is not for the short term. So
there needs to be something that is supportable on both u-boot and
grub (or any other bootloader).

>
>> We need to back-up and consider what this looks like in the end for
>> all the pieces and get input from folks on grub, UEFI, and armv8. The
>> UEFI answer may be this is a grub problem. For armv8, this proposal
>> does match up well as the kernel boot interface for v8 is DT. Despite
>> some claims, ACPI will not completely replace DT because of this.
>
> Yes, for UEFI it is absolutely an OS loader problem. UEFI provides an
> API and runtime environment. Grub is in general moving towards a boot
> menu system and a tool for loading images. Actual booting however
> should be done by a separate OS loader application. For Linux, this
> will be an in-kernel UEFI Stub. For Xen I would recommend taking the
> Linux EFI stub code and doing the same thing. There really isn't a
> need for a multiboot spec when you can rely on a runtime execution
> environment for setting things up exactly as you want them.

You've lost me as well. How do you see the flow working with UEFI for
a user running bare metal OS, installing Xen, and rebooting running
Xen.

Rob
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Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Grant Likely]

On Fri, 13 Sep 2013 20:40:54 -0500, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> On Fri, Sep 13, 2013 at 5:13 AM, Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org> wrote:
> > On Wed, Sep 4, 2013 at 5:41 PM, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >>> For u-boot Andre has proposed some syntactic sugar over the "fdt"
> >>> command to make boot.scr more trivial to use. We would of course need to
> >>> implement support for using it in the relevant distro tools (but they
> >>> tend to be very distro/machine specific already, e.g. Debian's
> >>> flash-kernel)
> >>
> >> And being machine specific is a PITA. flash-kernel is certainly not
> >> something we want to expand on. There is not much love for boot.scr
> >> either. There is work to address what are not really machine
> >> differences, but largely vendor u-boot differences:
> >>
> >> http://www.mail-archive.com/u-boot-0aAXYlwwYIKGBzrmiIFOJg@public.gmane.org/msg119025.html
> >>
> >> One option for u-boot which already supports syslinux style menu files
> >> is to adopt the syslinux multiboot parsing support:
> >>
> >> http://www.syslinux.org/wiki/index.php/Doc/mboot
> >
> > Even building it into U-Boot is problematic because it leaves older
> > machines out in the cold. Leif's port of Grub to U-Boot is far more
> > interesting since the distro can now be in control of the code that
> > loads the images and jumps into the kernel/hypervisor.
> 
> Considering there is no distro support for grub on ARM yet, it may be
> more interesting in the long run, but it is not for the short term. So
> there needs to be something that is supportable on both u-boot and
> grub (or any other bootloader)

True.

> 
> >
> >> We need to back-up and consider what this looks like in the end for
> >> all the pieces and get input from folks on grub, UEFI, and armv8. The
> >> UEFI answer may be this is a grub problem. For armv8, this proposal
> >> does match up well as the kernel boot interface for v8 is DT. Despite
> >> some claims, ACPI will not completely replace DT because of this.
> >
> > Yes, for UEFI it is absolutely an OS loader problem. UEFI provides an
> > API and runtime environment. Grub is in general moving towards a boot
> > menu system and a tool for loading images. Actual booting however
> > should be done by a separate OS loader application. For Linux, this
> > will be an in-kernel UEFI Stub. For Xen I would recommend taking the
> > Linux EFI stub code and doing the same thing. There really isn't a
> > need for a multiboot spec when you can rely on a runtime execution
> > environment for setting things up exactly as you want them.
> 
> You've lost me as well. How do you see the flow working with UEFI for
> a user running bare metal OS, installing Xen, and rebooting running
> Xen.

I see I've caused confusion. I wasn't disputing the proposed binding,
but rather stating that none of the Xen or Linux specific boot code
should be built directly into UEFI. It should be contained in a separate
UEFI OS Loader applications (GRUB + stub).

As for the question above, I see the Xen installer adding stanzas to the
Grub configuration to boo the xen kernel instead of the Linux kernel.
Here there are two options (the Xen folks can decide which is best)
a) The Xen kernel has an EFI stub which uses a "linux=" argument to
specify the Linux kernel image to load.
b) GRUB is Xen-aware and adds the extra images to the dtb.

In either case, the Xen loader is completely separate from UEFI and
therefore under our own control.

g.
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Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Dennis Gilmore]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

El Wed, 4 Sep 2013 11:41:01 -0500
Rob Herring <robherring2@gmail.com> escribió:
> Adding Dennis for a distro perspective.
> 
> On Wed, Sep 4, 2013 at 3:44 AM, Ian Campbell
> <Ian.Campbell@citrix.com> wrote:
> > On Tue, 2013-09-03 at 17:00 -0500, Rob Herring wrote:
> >> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara
> >> <andre.przywara@linaro.org> wrote:
> >> > Hi,
> >> >
> >> > a normal Linux kernel currently supports reading the start and
> >> > end address of a single binary blob via the FDT's /chosen node.
> >> > This will be interpreted as the location of an initial RAM disk.
> >> >
> >> > The Xen hypervisor itself is a kernel, but needs up to _two_
> >> > binaries for proper operation: a Dom0 Linux kernel and it's
> >> > associated initrd. On x86 this is solved via the multiboot
> >> > protocol used by the Grub bootloader, which supports to pass an
> >> > arbitrary number of binary modules to any kernel.
> >> >
> >> > Since in the ARM world we have the versatile device tree, we
> >> > don't need to implement the mulitboot protocol.
> >>
> >> But surely there would be some advantage of reuse by using the
> >> multi-boot protocol since Xen, grub, and OS tools already support
> >> it for x86.
> >
> > Multiboot is pretty x86 specific (although MB2 has a MIPS port) and
> > covers more stuff than we strictly require (e.g. on x86 it has
> > requirements around which processor mode you enter in, has paging
> > enabled etc).
> >
> >> > So I'd like to propose a new binding which denotes binary
> >> > modules a kernel can use at it's own discretion.
> >> > The need is triggered by the Xen hypervisor (which already uses
> >> > a very similar scheme), but the approach is deliberately chosen
> >> > to be as generic as possible to allow future uses (like passing
> >> > firmware blobs for devices or the like).
> >> > Credits for this go to Ian Campbell, who started something very
> >> > similar [1] for the Xen hypervisor. The intention of this
> >> > proposal is to make this generic and publicly documented.
> >>
> >> Can you describe how you see the boot flow working starting with OS
> >> installer writes kernel, initrd, xen and ??? to disk.
> >
> > Kernel and initrd are written to /boot in the usual way (probably
> > from kernel.deb or whatever). Xen would also normally come from a
> > distro package (also in /boot).
> >
> >> How does the bootloader know what to load?
> >
> > It's in the bootloader config, e.g. boot.scr or grub.cfg, which are
> > either hand written or produced by the distros tooling.
> >
> > grub on ARM could consume the same stanzas as are used by grub on
> > x86 to boot Xen (which are produced by update-grub):
> >         echo    'Loading Xen 4.1-amd64 ...'
> >         multiboot       /xen-4.1-amd64.gz placeholder
> >         echo    'Loading Linux 3.10-2-amd64 ...'
> >         module  /vmlinuz-3.10-2-amd64 placeholder
> > root=/dev/mapper/disks-root ro resume=/dev/mapper/disks-swap quiet
> > echo    'Loading initial ramdisk ...'
> > module  /initrd.img-3.10-2-amd64
> >
> > Since there is no multiboot on ARM (and never will be) this is safe.
> >
> > If multiboot ever does come to ARM it will necessarily be multiboot2
> > which uses a different keyword.
> 
> Right, this is just a text file with a list of binaries. It is not
> really the multiboot spec. There is no reason for this part to be
> different for grub on ARM. There is a big advantage to reusing the
> distro side tooling. If there isn't really much reuse on the
> bootloader side, then I'm fine with a different bootloader to Xen
> interface. I would like to hear that from folks working on grub
> though.
> 
> > For u-boot Andre has proposed some syntactic sugar over the "fdt"
> > command to make boot.scr more trivial to use. We would of course
> > need to implement support for using it in the relevant distro tools
> > (but they tend to be very distro/machine specific already, e.g.
> > Debian's flash-kernel)
> 
> And being machine specific is a PITA. flash-kernel is certainly not
> something we want to expand on. There is not much love for boot.scr
> either. There is work to address what are not really machine
> differences, but largely vendor u-boot differences:
> 
> http://www.mail-archive.com/u-boot@lists.denx.de/msg119025.html

u-boot still has quite a bit of work that needs to be done to make
things standard across the board. I hope we will get there. but it will
require vendors to update their u-boot builds. even for grub to be a
viable option u-boot needs updated.

> One option for u-boot which already supports syslinux style menu files
> is to adopt the syslinux multiboot parsing support:
> 
> http://www.syslinux.org/wiki/index.php/Doc/mboot

I would like to have u-boot support more of syslinux than it does today.

> 
> We need to back-up and consider what this looks like in the end for
> all the pieces and get input from folks on grub, UEFI, and armv8. The
> UEFI answer may be this is a grub problem. For armv8, this proposal
> does match up well as the kernel boot interface for v8 is DT. Despite
> some claims, ACPI will not completely replace DT because of this.
> 
> Rob

grub on arm still needs a bit of work. as does u-boot. ultimately
having things look the same between x86 and arm for the user is the
best option. so if u-boot supported syslinux's mboot spec and grub
supported the same syntax in both cases, regardless of if the
implementation varied wildly, the API for lack of a better word should
be consistent.

Dennis
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Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Grant Likely]

On Tue, 03 Sep 2013 17:53:44 +0200, Andre Przywara <andre.przywara-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> Hi,
> 
> a normal Linux kernel currently supports reading the start and end 
> address of a single binary blob via the FDT's /chosen node.
> This will be interpreted as the location of an initial RAM disk.
> 
> The Xen hypervisor itself is a kernel, but needs up to _two_ binaries 
> for proper operation: a Dom0 Linux kernel and it's associated initrd.
> On x86 this is solved via the multiboot protocol used by the Grub 
> bootloader, which supports to pass an arbitrary number of binary modules 
> to any kernel.
> 
> Since in the ARM world we have the versatile device tree, we don't need 
> to implement the mulitboot protocol.
> 
> So I'd like to propose a new binding which denotes binary modules a 
> kernel can use at it's own discretion.
> The need is triggered by the Xen hypervisor (which already uses a very 
> similar scheme), but the approach is deliberately chosen to be as 
> generic as possible to allow future uses (like passing firmware blobs 
> for devices or the like).
> Credits for this go to Ian Campbell, who started something very similar 
> [1] for the Xen hypervisor. The intention of this proposal is to make 
> this generic and publicly documented.
> 
> Looking forward to any comments!
> 
> Thanks,
> Andre.
> 
> [1] 
> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/arm/device-tree/booting.txt;h=94cd3f18a4e1317a35e1255bf5c6e1e091001d1a;hb=HEAD
> ----------------------------
> * Multiple boot modules device tree bindings
> 
> Boot loaders wanting to pass multiple additional binaries to a kernel 
> shall add a node "module" for each binary blob under the /chosen node 
> with the following properties:
> 
> - compatible:
>      compatible = "boot,module";
>    A bootloader may add names to more specifically describe the module,
>    e.g. Xen may use "xen,dom0-kernel" or "xen,dom0-ramdisk".
>    If possible a kernel should be able to use modules even without a
>    descriptive naming, by enumerating them in order and using hard-coded
>    meanings for each module (e.g. first is kernel, second is initrd).
> 
> - reg: specifies the base physical address and size of a region in
>    memory where the bootloader loaded the respective binary data to.
> 
> - bootargs:
>    An optional property describing arguments to use for this module.
>    Could be a command line or configuration data.
> 
> Example:
> /chosen {
>      #size-cells = <0x1>;
>      #address-cells = <0x1>;
>      module@0 {
>          compatible = "xen,linux-zimage", "xen,multiboot-module", 
> "boot,module";
>          reg = <0x80000000 0x003dcff8>;
>          bootargs = "console=hvc0 earlyprintk ro root=/dev/sda1 nosmp";
>      };
>      module@1 {
>          compatible = "xen,linux-initrd", "xen,multiboot-module", 
> "boot,module";
>          reg = <0x08000000 0x00123456>;
>      };

The moment you pull in a 'reg' property, your required to property parse
#size-cells and #address-cells and also 'ranges' which is missing from
the above. I don't necessarily object to that, but it can be a lot to
ask for during early boot. It also looks really weird in the chosen
node. For boot time artifacts like this, I'd rather see a duplication of
the linux,initrd- pattern. Something like:

/chosen {
	xen,linux-zimage-start = <0x80000000>;
	xen,linux-zimage-end = <0x803dcff8>;
	linux,initrd-start = <0x8000000>;
	linux,initrd-end = <0x8123456>;
}

If have two reason for this; consistency and simplicity. Consistent
because it matches what Linux already uses, and really easy to parse
without mucking with ranges or #address/size-cells. The assumption is
that during early boot the system is dealing with a 1:1 mapped physical
address and there isn't any bus translations that need to be processed.

Question: what is the need for the xen,linux-initrd node? Why can't the
existing "linux,initrd-*" properties continue to be used?

g.

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Re: [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Andre Przywara]

On 09/15/2013 02:37 PM, Grant Likely wrote:
> On Tue, 03 Sep 2013 17:53:44 +0200, Andre Przywara <andre.przywara-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>> Hi,
>>
>> a normal Linux kernel currently supports reading the start and end
>> address of a single binary blob via the FDT's /chosen node.
>> This will be interpreted as the location of an initial RAM disk.
>>
>> The Xen hypervisor itself is a kernel, but needs up to _two_ binaries
>> for proper operation: a Dom0 Linux kernel and it's associated initrd.
>> On x86 this is solved via the multiboot protocol used by the Grub
>> bootloader, which supports to pass an arbitrary number of binary modules
>> to any kernel.
>>
>> Since in the ARM world we have the versatile device tree, we don't need
>> to implement the mulitboot protocol.
>>
>> So I'd like to propose a new binding which denotes binary modules a
>> kernel can use at it's own discretion.
>> The need is triggered by the Xen hypervisor (which already uses a very
>> similar scheme), but the approach is deliberately chosen to be as
>> generic as possible to allow future uses (like passing firmware blobs
>> for devices or the like).
>> Credits for this go to Ian Campbell, who started something very similar
>> [1] for the Xen hypervisor. The intention of this proposal is to make
>> this generic and publicly documented.
>>
>> Looking forward to any comments!
>>
>> Thanks,
>> Andre.
>>
>> [1]
>> http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/arm/device-tree/booting.txt;h=94cd3f18a4e1317a35e1255bf5c6e1e091001d1a;hb=HEAD
>> ----------------------------
>> * Multiple boot modules device tree bindings
>>
>> Boot loaders wanting to pass multiple additional binaries to a kernel
>> shall add a node "module" for each binary blob under the /chosen node
>> with the following properties:
>>
>> - compatible:
>>       compatible = "boot,module";
>>     A bootloader may add names to more specifically describe the module,
>>     e.g. Xen may use "xen,dom0-kernel" or "xen,dom0-ramdisk".
>>     If possible a kernel should be able to use modules even without a
>>     descriptive naming, by enumerating them in order and using hard-coded
>>     meanings for each module (e.g. first is kernel, second is initrd).
>>
>> - reg: specifies the base physical address and size of a region in
>>     memory where the bootloader loaded the respective binary data to.
>>
>> - bootargs:
>>     An optional property describing arguments to use for this module.
>>     Could be a command line or configuration data.
>>
>> Example:
>> /chosen {
>>       #size-cells = <0x1>;
>>       #address-cells = <0x1>;
>>       module@0 {
>>           compatible = "xen,linux-zimage", "xen,multiboot-module",
>> "boot,module";
>>           reg = <0x80000000 0x003dcff8>;
>>           bootargs = "console=hvc0 earlyprintk ro root=/dev/sda1 nosmp";
>>       };
>>       module@1 {
>>           compatible = "xen,linux-initrd", "xen,multiboot-module",
>> "boot,module";
>>           reg = <0x08000000 0x00123456>;
>>       };

Grant,

thanks for looking at this. Appreciate your comments.

> The moment you pull in a 'reg' property, your required to property parse
> #size-cells and #address-cells and also 'ranges' which is missing from
> the above.

You mean that early boot code has to read #size-cells and #address-cells 
and act accordingly when parsing the reg property? So just more code in 
early boot? But that would easily open the door for 64bit addresses for 
instance, right?

> I don't necessarily object to that, but it can be a lot to
> ask for during early boot.

I don't think Xen in particular has a lot of problems with that. The DT 
implementation is quite capable (see Julien's latest updates).

 > It also looks really weird in the chosen node.

TBH this initrd syntax looks a bit weird to me in general, not following 
the nice and well defined DT way (no compatible, separate properties for 
start and length, ...)

> For boot time artifacts like this, I'd rather see a duplication of
> the linux,initrd- pattern. Something like:
> /chosen {
> 	xen,linux-zimage-start = <0x80000000>;
> 	xen,linux-zimage-end = <0x803dcff8>;
> 	linux,initrd-start = <0x8000000>;
> 	linux,initrd-end = <0x8123456>;
> }

To me that sounds like a hack to avoid more coding work.
The problem with that is that it would introduce a Xen specific property 
(actually two).
So from a user's perspective you want:

a) generic properties to support future OSes without much hassle (using 
the DT's compatible semantic)
b) specifying load address and length in the normal way
c) adding command lines
d) potentially adding more arbitrary properties
e) allowing an arbitrary number of modules

To just go ahead with another hard-coded property to make life easier 
for us kernel level developers doesn't sound very convincing to me.
And especially supporting multiple modules would be very hackish then.

> If have two reason for this; consistency and simplicity. Consistent
> because it matches what Linux already uses,

But this is actually not consistent with the rest of the DT 
architecture. Defining an own node for that looks like a perfect match 
to me in the DT world.

> and really easy to parse
> without mucking with ranges or #address/size-cells. The assumption is
> that during early boot the system is dealing with a 1:1 mapped physical
> address and there isn't any bus translations that need to be processed.

I see your point with early boot code constraints. But my idea was to 
create a flexible and generic solution, so that there will be a 
framework for bootloaders to specify blobs without needing updates every 
time someone comes up with a need for one. On x86 there was once such a 
need for microcode updates which couldn't be easily satisfied because 
Linux/x86 itself does not support the multiboot protocol.

> Question: what is the need for the xen,linux-initrd node? Why can't the
> existing "linux,initrd-*" properties continue to be used?

The main driver for this was to avoid patching bootloaders just to boot 
Xen specifically. Rather we want to patch bootloaders to support some 
kind of "multiboot" protocol once and let Xen piggy-back on this. I 
think that is somewhat driven by experiences in the x86 world, were 
especially kernel people are reluctant to support something just for 
Xen's need.

Regards,
Andre.

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