Creating a wrapped zImage.initrd -- can't start the trampoline?

Creating a wrapped zImage.initrd -- can't start the trampoline?

[Paul Smith]

Hi all;
        
I'm working with a PPC 460EX-based board.  I have a 2.6.25.10-based
kernel I obtained from the DENX git tree which contains a port to this
board, under arch/powerpc.

I can build a kernel and have it downloaded and booting using u-boot,
providing the blob and appropriate kernel parameters, after having done
a standard "make zImage" with the appropriate configs etc.  All good so
far.

However, my ultimate target board uses a custom bootloader, not uboot,
which is very, very simple (please don't suggest we just switch to
uboot--that argument has been already had, and lost :-/)  I had a
previous version of Linux, 2.6.14, booting on a predecessor of this
board based on the 440GX with the same bootloader, but the creation of
that image was done under arch/ppc and it was pretty different (I'm
discovering).  And, I didn't do that port anyway.

Anyway, I need to generate an all-in-one image containing the kernel and
the initrd, and the trampoline code tacked onto it to turn it into an
ELF image.  I'm worried I'll need to write a custom platform_init() but
for now I'm just using of.c.

>From what I can tell, the wrapper script seems to be intended to have
the ability to generate an image like this.  I've tried just running
"make zImage.initrd" after putting the ramdisk.initrd.gz in the right
place, and it seems to create an image OK: using file shows it to be
"ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV),
statically linked, not stripped", which seems correct.

But we can't get it to boot.

The bootloader downloads the image (tftp) and jumps to the start address
in the ELF image, and immediately takes an illegal instruction.  We have
set the bootloader to load the image at 0x00000000 and then we look
through the ELF image for the start address, which (using objdump) I can
see is the correct address for the _zimage_start symbol from crt0.S; in
our case it's 0x00400204.

When we use a debugger to look at what's loaded at that location, it's
just a whole slew of 0xa bytes... obviously not right.  Is the start
address not the offset from the start of the ELF image?  Should we be
loading the image somewhere else (the previous incarnation loaded it at
0x0, so we just did that too)?

I'm afraid I'm at the very outer limits of my knowledge on these
matters.  I realize there is not enough detail in the above to really
provide an answer but any suggestions anyone has would be very much
appreciated.

Cheers!

Re: Creating a wrapped zImage.initrd -- can't start the trampoline?

[Josh Boyer]

On Fri, 2008-08-01 at 22:40 -0400, Paul Smith wrote:
> Hi all;
>         
> I'm working with a PPC 460EX-based board.  I have a 2.6.25.10-based
> kernel I obtained from the DENX git tree which contains a port to this
> board, under arch/powerpc.
> 
> I can build a kernel and have it downloaded and booting using u-boot,
> providing the blob and appropriate kernel parameters, after having done
> a standard "make zImage" with the appropriate configs etc.  All good so
> far.
> 
> However, my ultimate target board uses a custom bootloader, not uboot,
> which is very, very simple (please don't suggest we just switch to
> uboot--that argument has been already had, and lost :-/)  I had a
> previous version of Linux, 2.6.14, booting on a predecessor of this
> board based on the 440GX with the same bootloader, but the creation of
> that image was done under arch/ppc and it was pretty different (I'm
> discovering).  And, I didn't do that port anyway.
> 
> Anyway, I need to generate an all-in-one image containing the kernel and
> the initrd, and the trampoline code tacked onto it to turn it into an
> ELF image.  I'm worried I'll need to write a custom platform_init() but
> for now I'm just using of.c.

It needs the vmlinux, dtb, and initrd in a single image.  The dtb is
required for arch/powerpc ports.

> >From what I can tell, the wrapper script seems to be intended to have
> the ability to generate an image like this.  I've tried just running
> "make zImage.initrd" after putting the ramdisk.initrd.gz in the right
> place, and it seems to create an image OK: using file shows it to be
> "ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV),
> statically linked, not stripped", which seems correct.
> 
> But we can't get it to boot.
> 
> The bootloader downloads the image (tftp) and jumps to the start address
> in the ELF image, and immediately takes an illegal instruction.  We have
> set the bootloader to load the image at 0x00000000 and then we look
> through the ELF image for the start address, which (using objdump) I can
> see is the correct address for the _zimage_start symbol from crt0.S; in
> our case it's 0x00400204.
> 
> When we use a debugger to look at what's loaded at that location, it's
> just a whole slew of 0xa bytes... obviously not right.  Is the start
> address not the offset from the start of the ELF image?  Should we be
> loading the image somewhere else (the previous incarnation loaded it at
> 0x0, so we just did that too)?

Load it at the link address.  Which is 4MiB.  A normal bootloader would
have done that.  Then the wrapper code will uncompress your kernel to
0x0 for you.

josh

Re: Creating a wrapped zImage.initrd -- can't start the trampoline?

[Paul Smith]

On Sat, 2008-08-02 at 08:35 -0400, Josh Boyer wrote:

> It needs the vmlinux, dtb, and initrd in a single image.  The dtb is
> required for arch/powerpc ports.

We have a dtb that we were using with uboot.  We were hoping we could
continue to do that even with the trampoline: that is, get the dtb from
the bootloader, while leaving the kernel and initrd in the downloaded
image.  We have coded up the bootloader to provide this information on
the command line and I was kind of hoping the trampoline would just pass
it through when it booted the uncompressed kernel.

It just seems to make more sense to have the dtb tied to the
hardware/firmware.

Is this possible?  If not we can do it the other way around (I found the
right targets in the Linux build system for this, I think).

> > The bootloader downloads the image (tftp) and jumps to the start address
> > in the ELF image, and immediately takes an illegal instruction.  We have
> > set the bootloader to load the image at 0x00000000 and then we look
> > through the ELF image for the start address, which (using objdump) I can
> > see is the correct address for the _zimage_start symbol from crt0.S; in
> > our case it's 0x00400204.
> > 
> > When we use a debugger to look at what's loaded at that location, it's
> > just a whole slew of 0xa bytes... obviously not right.  Is the start
> > address not the offset from the start of the ELF image?  Should we be
> > loading the image somewhere else (the previous incarnation loaded it at
> > 0x0, so we just did that too)?
> 
> Load it at the link address.  Which is 4MiB.  A normal bootloader would
> have done that.  Then the wrapper code will uncompress your kernel to
> 0x0 for you.

Ah.  OK, that's good info.  We'll give that a go.

Thanks!

Re: Creating a wrapped zImage.initrd -- can't start the trampoline?

[Grant Likely]

On Fri, Aug 01, 2008 at 10:40:27PM -0400, Paul Smith wrote:

> Anyway, I need to generate an all-in-one image containing the kernel and
> the initrd, and the trampoline code tacked onto it to turn it into an
> ELF image.  I'm worried I'll need to write a custom platform_init() but
> for now I'm just using of.c.

Take a look at Documentation/powerpc/bootwrapper.txt

You probably want the 'simpleImage.%' target.  It also binds in a copy
of the device tree blob.

g.

Re: Creating a wrapped zImage.initrd -- can't start the trampoline?

[Grant Likely]

On Mon, Aug 4, 2008 at 1:43 PM, Paul Smith <paul@mad-scientist.us> wrote:
> On Mon, 2008-08-04 at 11:25 -0600, Grant Likely wrote:
>> Take a look at Documentation/powerpc/bootwrapper.txt
>> You probably want the 'simpleImage.%' target.  It also binds in a copy
>> of the device tree blob.
>
> Hm.  I'm using 2.6.25.14 and I don't see any bootwrapper.txt file in
> Documentation/powerpc (or anywhere else).  I also can't see any target
> simpleImage in any makefile.  Is this something new in 2.6.26?  :-(
>
> Any help for folks still using 2.6.25?

It's new in .26, but it is relevant for .25 also.

Here's a link to it:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/powerpc/bootwrapper.txt;h=d60fced5e1cc69169d9bbf1b49faacb41ef8db9c;hb=8f616cd5249e03c9e1b371623d85e76d4b86bbc1

> I think I need some basic instruction on how bootloaders etc. work, with
> a concentration on embedded PPC if possible.  Anyone have any pointers
> that would be worth investigating?

The document above should help.
Documentation/powerpc/booting-without-of.txt has some useful
information also.

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.