Getting rid of falcon mode

Getting rid of falcon mode

[Alex G.]

## Introduction

Today we use "falcon mode" to mean "boot linux straight from SPL". This 
designation makes sense, since falcons "fly at high speed and change 
direction rapidly" according to Wikipedia.

The way we implement falcon mode is to reserve two areas of storage:
   * kernel area/partition
   * dtb area/partition
By using some "special cases", and "spl export", SPL can more or less 
figure out how to skip u-boot.


## The plot twist

People familiar with FIT, will have recognized that the above is 
achievable with a very basic FIT image. With some advantages:

     - No "special cases" in SPL code
     - Signed kernel images
     - Signed kernel devicetree
     - Devicetree overlays
     - Automatic selection of correct devicetree


## The problems

The advantages of FIT are not obvious by looking at SPL code. A 
noticeable amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, 
leading one to believe that SPL_OS_BOOT is very important. It must be 
since it takes up so much code.

Enabling falcon mode is not well documented, and requires a lot of trial 
and error. I've had to define 7 macros, and one new function to get it 
working on my board -- and vividly remember the grief. This is an 
antiquated way of doing things, and completely ignores the u-boot 
devicetree -- we could just as well have defined those seven values in 
the dtb.

SPL assumes that it must load u-boot, unless in instances under 
CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and 
confusion over the past several months. I have no less than three patch 
series trying to address shortfalls there. It's awful.


## The proposal

I propose we drop falcon mode support for legacy images.

   - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
   - Drop the "dtb area/partition". The dtb is derived from the FIT
   - Drop "spl export"


How do we deal with devicetrees in the FIT then? The options are to use 
a modified devicetree which has the desired "/chosen" node, or use DTB 
overlays.

What are the reasons why we shouldn't go this route?

Alex

Getting rid of falcon mode

[Tom Rini]

On Mon, Apr 12, 2021 at 04:32:49PM -0500, Alex G. wrote:
> ## Introduction
> 
> Today we use "falcon mode" to mean "boot linux straight from SPL". This
> designation makes sense, since falcons "fly at high speed and change
> direction rapidly" according to Wikipedia.
> 
> The way we implement falcon mode is to reserve two areas of storage:
>   * kernel area/partition
>   * dtb area/partition
> By using some "special cases", and "spl export", SPL can more or less figure
> out how to skip u-boot.
> 
> 
> ## The plot twist
> 
> People familiar with FIT, will have recognized that the above is achievable
> with a very basic FIT image. With some advantages:
> 
>     - No "special cases" in SPL code
>     - Signed kernel images
>     - Signed kernel devicetree
>     - Devicetree overlays
>     - Automatic selection of correct devicetree
> 
> 
> ## The problems
> 
> The advantages of FIT are not obvious by looking at SPL code. A noticeable
> amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, leading one to
> believe that SPL_OS_BOOT is very important. It must be since it takes up so
> much code.
> 
> Enabling falcon mode is not well documented, and requires a lot of trial and
> error. I've had to define 7 macros, and one new function to get it working
> on my board -- and vividly remember the grief. This is an antiquated way of
> doing things, and completely ignores the u-boot devicetree -- we could just
> as well have defined those seven values in the dtb.
> 
> SPL assumes that it must load u-boot, unless in instances under
> CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and confusion
> over the past several months. I have no less than three patch series trying
> to address shortfalls there. It's awful.
> 
> 
> ## The proposal
> 
> I propose we drop falcon mode support for legacy images.
> 
>   - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
>   - Drop the "dtb area/partition". The dtb is derived from the FIT
>   - Drop "spl export"
> 
> 
> How do we deal with devicetrees in the FIT then? The options are to use a
> modified devicetree which has the desired "/chosen" node, or use DTB
> overlays.
> 
> What are the reasons why we shouldn't go this route?

Please send an RFC that shows what doc/usage/falcon.rst would look like
(base it as much or as little on doc/README.falon as you like).
Conceptually, yes, this sounds like the right way to go moving forward.
It should just be removing code, except for around the "boot to U-Boot
not OS" stuff.  Thanks!

-- 
Tom
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 659 bytes
Desc: not available
URL: <https://lists.denx.de/pipermail/u-boot/attachments/20210412/9b20de0a/attachment.sig>

Getting rid of falcon mode

[Maxime Ripard]

Hi,

On Mon, Apr 12, 2021 at 04:32:49PM -0500, Alex G. wrote:
> ## Introduction
> 
> Today we use "falcon mode" to mean "boot linux straight from SPL". This
> designation makes sense, since falcons "fly at high speed and change
> direction rapidly" according to Wikipedia.
> 
> The way we implement falcon mode is to reserve two areas of storage:
>   * kernel area/partition
>   * dtb area/partition
> By using some "special cases", and "spl export", SPL can more or less figure
> out how to skip u-boot.
> 
> 
> ## The plot twist
> 
> People familiar with FIT, will have recognized that the above is achievable
> with a very basic FIT image. With some advantages:
> 
>     - No "special cases" in SPL code
>     - Signed kernel images
>     - Signed kernel devicetree
>     - Devicetree overlays
>     - Automatic selection of correct devicetree
> 
> 
> ## The problems
> 
> The advantages of FIT are not obvious by looking at SPL code. A noticeable
> amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, leading one to
> believe that SPL_OS_BOOT is very important. It must be since it takes up so
> much code.
> 
> Enabling falcon mode is not well documented, and requires a lot of trial and
> error. I've had to define 7 macros, and one new function to get it working
> on my board -- and vividly remember the grief. This is an antiquated way of
> doing things, and completely ignores the u-boot devicetree -- we could just
> as well have defined those seven values in the dtb.
> 
> SPL assumes that it must load u-boot, unless in instances under
> CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and confusion
> over the past several months. I have no less than three patch series trying
> to address shortfalls there. It's awful.
> 
> 
> ## The proposal
> 
> I propose we drop falcon mode support for legacy images.
> 
>   - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
>   - Drop the "dtb area/partition". The dtb is derived from the FIT
>   - Drop "spl export"
> 
> 
> How do we deal with devicetrees in the FIT then? The options are to use a
> modified devicetree which has the desired "/chosen" node, or use DTB
> overlays.
> 
> What are the reasons why we shouldn't go this route?

I can see at least two, that are mainly due to a FIT image being
essentially a compiled device tree:

  - Not all platforms have enough head-space in their SPL to have libfdt
    in addition to what is already there.

  - Parsing a DT is fairly slow too. Last time I checked booting a FIT
    image took around 150ms more than a legacy image on a Cortex-A7. If
    one is using the Falcon mode in the first place chances are it's to
    improve the boot-time, so this seems like a step backward.

Maxime
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL: <https://lists.denx.de/pipermail/u-boot/attachments/20210413/3d7d280f/attachment.sig>

Getting rid of falcon mode

[Alex G.]

Hi Maxime,

On 4/13/21 3:56 AM, Maxime Ripard wrote:
> Hi,
> 
> On Mon, Apr 12, 2021 at 04:32:49PM -0500, Alex G. wrote:
>> ## Introduction
>>
>> Today we use "falcon mode" to mean "boot linux straight from SPL". This
>> designation makes sense, since falcons "fly at high speed and change
>> direction rapidly" according to Wikipedia.
>>
>> The way we implement falcon mode is to reserve two areas of storage:
>>    * kernel area/partition
>>    * dtb area/partition
>> By using some "special cases", and "spl export", SPL can more or less figure
>> out how to skip u-boot.
>>
>>
>> ## The plot twist
>>
>> People familiar with FIT, will have recognized that the above is achievable
>> with a very basic FIT image. With some advantages:
>>
>>      - No "special cases" in SPL code
>>      - Signed kernel images
>>      - Signed kernel devicetree
>>      - Devicetree overlays
>>      - Automatic selection of correct devicetree
>>
>>
>> ## The problems
>>
>> The advantages of FIT are not obvious by looking at SPL code. A noticeable
>> amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, leading one to
>> believe that SPL_OS_BOOT is very important. It must be since it takes up so
>> much code.
>>
>> Enabling falcon mode is not well documented, and requires a lot of trial and
>> error. I've had to define 7 macros, and one new function to get it working
>> on my board -- and vividly remember the grief. This is an antiquated way of
>> doing things, and completely ignores the u-boot devicetree -- we could just
>> as well have defined those seven values in the dtb.
>>
>> SPL assumes that it must load u-boot, unless in instances under
>> CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and confusion
>> over the past several months. I have no less than three patch series trying
>> to address shortfalls there. It's awful.
>>
>>
>> ## The proposal
>>
>> I propose we drop falcon mode support for legacy images.
>>
>>    - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
>>    - Drop the "dtb area/partition". The dtb is derived from the FIT
>>    - Drop "spl export"
>>
>>
>> How do we deal with devicetrees in the FIT then? The options are to use a
>> modified devicetree which has the desired "/chosen" node, or use DTB
>> overlays.
>>
>> What are the reasons why we shouldn't go this route?
> 
> I can see at least two, that are mainly due to a FIT image being
> essentially a compiled device tree:
> 
>    - Not all platforms have enough head-space in their SPL to have libfdt
>      in addition to what is already there.

Do we know which platforms fall in this category? We can investigate if 
it might be possible to disable just enough of the legacy support to 
make room for libfdt.


>    - Parsing a DT is fairly slow too. Last time I checked booting a FIT
>      image took around 150ms more than a legacy image on a Cortex-A7. If
>      one is using the Falcon mode in the first place chances are it's to
>      improve the boot-time, so this seems like a step backward.

I'm skeptical of the 150ms delta, as I also did heavy measurements on 
this a few months back. But maybe there's something that causes certain 
platforms to bog down, (caching issues ?).

I've used grabserial (e.g. "grabserial -d /dev/ttyACM0 -b 2000000 
-m"U-Boot SPL" -t") to time the boot. If you have similar logs, maybe 
there's something there that tells us why parsing the FIT bogs down.

Alex

Getting rid of falcon mode

[Tom Rini]

On Tue, Apr 13, 2021 at 09:08:01AM -0500, Alex G. wrote:
> Hi Maxime,
> 
> On 4/13/21 3:56 AM, Maxime Ripard wrote:
> > Hi,
> > 
> > On Mon, Apr 12, 2021 at 04:32:49PM -0500, Alex G. wrote:
> > > ## Introduction
> > > 
> > > Today we use "falcon mode" to mean "boot linux straight from SPL". This
> > > designation makes sense, since falcons "fly at high speed and change
> > > direction rapidly" according to Wikipedia.
> > > 
> > > The way we implement falcon mode is to reserve two areas of storage:
> > >    * kernel area/partition
> > >    * dtb area/partition
> > > By using some "special cases", and "spl export", SPL can more or less figure
> > > out how to skip u-boot.
> > > 
> > > 
> > > ## The plot twist
> > > 
> > > People familiar with FIT, will have recognized that the above is achievable
> > > with a very basic FIT image. With some advantages:
> > > 
> > >      - No "special cases" in SPL code
> > >      - Signed kernel images
> > >      - Signed kernel devicetree
> > >      - Devicetree overlays
> > >      - Automatic selection of correct devicetree
> > > 
> > > 
> > > ## The problems
> > > 
> > > The advantages of FIT are not obvious by looking at SPL code. A noticeable
> > > amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, leading one to
> > > believe that SPL_OS_BOOT is very important. It must be since it takes up so
> > > much code.
> > > 
> > > Enabling falcon mode is not well documented, and requires a lot of trial and
> > > error. I've had to define 7 macros, and one new function to get it working
> > > on my board -- and vividly remember the grief. This is an antiquated way of
> > > doing things, and completely ignores the u-boot devicetree -- we could just
> > > as well have defined those seven values in the dtb.
> > > 
> > > SPL assumes that it must load u-boot, unless in instances under
> > > CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and confusion
> > > over the past several months. I have no less than three patch series trying
> > > to address shortfalls there. It's awful.
> > > 
> > > 
> > > ## The proposal
> > > 
> > > I propose we drop falcon mode support for legacy images.
> > > 
> > >    - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
> > >    - Drop the "dtb area/partition". The dtb is derived from the FIT
> > >    - Drop "spl export"
> > > 
> > > 
> > > How do we deal with devicetrees in the FIT then? The options are to use a
> > > modified devicetree which has the desired "/chosen" node, or use DTB
> > > overlays.
> > > 
> > > What are the reasons why we shouldn't go this route?
> > 
> > I can see at least two, that are mainly due to a FIT image being
> > essentially a compiled device tree:
> > 
> >    - Not all platforms have enough head-space in their SPL to have libfdt
> >      in addition to what is already there.
> 
> Do we know which platforms fall in this category? We can investigate if it
> might be possible to disable just enough of the legacy support to make room
> for libfdt.

sunxi is both modern and small, so a good thing to benchmark.

> >    - Parsing a DT is fairly slow too. Last time I checked booting a FIT
> >      image took around 150ms more than a legacy image on a Cortex-A7. If
> >      one is using the Falcon mode in the first place chances are it's to
> >      improve the boot-time, so this seems like a step backward.
> 
> I'm skeptical of the 150ms delta, as I also did heavy measurements on this a
> few months back. But maybe there's something that causes certain platforms
> to bog down, (caching issues ?).
> 
> I've used grabserial (e.g. "grabserial -d /dev/ttyACM0 -b 2000000 -m"U-Boot
> SPL" -t") to time the boot. If you have similar logs, maybe there's
> something there that tells us why parsing the FIT bogs down.

Since you have this working on some platforms, can you show some logs?
Sample configurations?  Thanks.

-- 
Tom
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 659 bytes
Desc: not available
URL: <https://lists.denx.de/pipermail/u-boot/attachments/20210413/02367a2f/attachment.sig>

Getting rid of falcon mode

[Maxime Ripard]

On Tue, Apr 13, 2021 at 09:08:01AM -0500, Alex G. wrote:
> On 4/13/21 3:56 AM, Maxime Ripard wrote:
> > On Mon, Apr 12, 2021 at 04:32:49PM -0500, Alex G. wrote:
> > > ## Introduction
> > > 
> > > Today we use "falcon mode" to mean "boot linux straight from SPL". This
> > > designation makes sense, since falcons "fly at high speed and change
> > > direction rapidly" according to Wikipedia.
> > > 
> > > The way we implement falcon mode is to reserve two areas of storage:
> > >    * kernel area/partition
> > >    * dtb area/partition
> > > By using some "special cases", and "spl export", SPL can more or less figure
> > > out how to skip u-boot.
> > > 
> > > 
> > > ## The plot twist
> > > 
> > > People familiar with FIT, will have recognized that the above is achievable
> > > with a very basic FIT image. With some advantages:
> > > 
> > >      - No "special cases" in SPL code
> > >      - Signed kernel images
> > >      - Signed kernel devicetree
> > >      - Devicetree overlays
> > >      - Automatic selection of correct devicetree
> > > 
> > > 
> > > ## The problems
> > > 
> > > The advantages of FIT are not obvious by looking at SPL code. A noticeable
> > > amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, leading one to
> > > believe that SPL_OS_BOOT is very important. It must be since it takes up so
> > > much code.
> > > 
> > > Enabling falcon mode is not well documented, and requires a lot of trial and
> > > error. I've had to define 7 macros, and one new function to get it working
> > > on my board -- and vividly remember the grief. This is an antiquated way of
> > > doing things, and completely ignores the u-boot devicetree -- we could just
> > > as well have defined those seven values in the dtb.
> > > 
> > > SPL assumes that it must load u-boot, unless in instances under
> > > CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and confusion
> > > over the past several months. I have no less than three patch series trying
> > > to address shortfalls there. It's awful.
> > > 
> > > 
> > > ## The proposal
> > > 
> > > I propose we drop falcon mode support for legacy images.
> > > 
> > >    - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
> > >    - Drop the "dtb area/partition". The dtb is derived from the FIT
> > >    - Drop "spl export"
> > > 
> > > 
> > > How do we deal with devicetrees in the FIT then? The options are to use a
> > > modified devicetree which has the desired "/chosen" node, or use DTB
> > > overlays.
> > > 
> > > What are the reasons why we shouldn't go this route?
> > 
> > I can see at least two, that are mainly due to a FIT image being
> > essentially a compiled device tree:
> > 
> >    - Not all platforms have enough head-space in their SPL to have libfdt
> >      in addition to what is already there.
> 
> Do we know which platforms fall in this category? We can investigate if it
> might be possible to disable just enough of the legacy support to make room
> for libfdt.

The earliest Allwinner SoC (A10, A13) must be fairly close to their
limit. I haven't checked for a while but they had 13kB overall iirc.

> >    - Parsing a DT is fairly slow too. Last time I checked booting a FIT
> >      image took around 150ms more than a legacy image on a Cortex-A7. If
> >      one is using the Falcon mode in the first place chances are it's to
> >      improve the boot-time, so this seems like a step backward.
> 
> I'm skeptical of the 150ms delta, as I also did heavy measurements on this a
> few months back. But maybe there's something that causes certain platforms
> to bog down, (caching issues ?).

I found my old notes back, and while the 150ms figure was correct, the
FIT parsing was only half that (the other one was actually patching the
DT to fixup whatever was needed).

80ms is still fairly expensive to just parse the image. I'm not sure
indeed whether sunxi has the MMU enabled in the SPL though. I think it
is but might very well be wrong.

> I've used grabserial (e.g. "grabserial -d /dev/ttyACM0 -b 2000000 -m"U-Boot
> SPL" -t") to time the boot. If you have similar logs, maybe there's
> something there that tells us why parsing the FIT bogs down.

Unfortunately this was part of my former job and don't have access to
them anymore :/

Maxime
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL: <https://lists.denx.de/pipermail/u-boot/attachments/20210413/bc561af7/attachment.sig>

Getting rid of falcon mode

[Maxime Ripard]

On Tue, Apr 13, 2021 at 10:13:50AM -0400, Tom Rini wrote:
> On Tue, Apr 13, 2021 at 09:08:01AM -0500, Alex G. wrote:
> > Hi Maxime,
> > 
> > On 4/13/21 3:56 AM, Maxime Ripard wrote:
> > > Hi,
> > > 
> > > On Mon, Apr 12, 2021 at 04:32:49PM -0500, Alex G. wrote:
> > > > ## Introduction
> > > > 
> > > > Today we use "falcon mode" to mean "boot linux straight from SPL". This
> > > > designation makes sense, since falcons "fly at high speed and change
> > > > direction rapidly" according to Wikipedia.
> > > > 
> > > > The way we implement falcon mode is to reserve two areas of storage:
> > > >    * kernel area/partition
> > > >    * dtb area/partition
> > > > By using some "special cases", and "spl export", SPL can more or less figure
> > > > out how to skip u-boot.
> > > > 
> > > > 
> > > > ## The plot twist
> > > > 
> > > > People familiar with FIT, will have recognized that the above is achievable
> > > > with a very basic FIT image. With some advantages:
> > > > 
> > > >      - No "special cases" in SPL code
> > > >      - Signed kernel images
> > > >      - Signed kernel devicetree
> > > >      - Devicetree overlays
> > > >      - Automatic selection of correct devicetree
> > > > 
> > > > 
> > > > ## The problems
> > > > 
> > > > The advantages of FIT are not obvious by looking at SPL code. A noticeable
> > > > amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT, leading one to
> > > > believe that SPL_OS_BOOT is very important. It must be since it takes up so
> > > > much code.
> > > > 
> > > > Enabling falcon mode is not well documented, and requires a lot of trial and
> > > > error. I've had to define 7 macros, and one new function to get it working
> > > > on my board -- and vividly remember the grief. This is an antiquated way of
> > > > doing things, and completely ignores the u-boot devicetree -- we could just
> > > > as well have defined those seven values in the dtb.
> > > > 
> > > > SPL assumes that it must load u-boot, unless in instances under
> > > > CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and confusion
> > > > over the past several months. I have no less than three patch series trying
> > > > to address shortfalls there. It's awful.
> > > > 
> > > > 
> > > > ## The proposal
> > > > 
> > > > I propose we drop falcon mode support for legacy images.
> > > > 
> > > >    - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
> > > >    - Drop the "dtb area/partition". The dtb is derived from the FIT
> > > >    - Drop "spl export"
> > > > 
> > > > 
> > > > How do we deal with devicetrees in the FIT then? The options are to use a
> > > > modified devicetree which has the desired "/chosen" node, or use DTB
> > > > overlays.
> > > > 
> > > > What are the reasons why we shouldn't go this route?
> > > 
> > > I can see at least two, that are mainly due to a FIT image being
> > > essentially a compiled device tree:
> > > 
> > >    - Not all platforms have enough head-space in their SPL to have libfdt
> > >      in addition to what is already there.
> > 
> > Do we know which platforms fall in this category? We can investigate if it
> > might be possible to disable just enough of the legacy support to make room
> > for libfdt.
> 
> sunxi is both modern and small, so a good thing to benchmark.

Another issue with sunxi is that it implements PSCI in U-Boot, so you'll
need in the SPL as well if you want to do falcon boot. This is doable on
the later SoCs that have a bigger SRAM, but for that kind of test you'll
want to test on the A10 or A13

Maxime
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL: <https://lists.denx.de/pipermail/u-boot/attachments/20210413/66058d5f/attachment.sig>

Getting rid of falcon mode

[Simon Glass]

Hi,

On Tue, 13 Apr 2021 at 09:32, Alex G. <mr.nuke.me@gmail.com> wrote:
>
> ## Introduction
>
> Today we use "falcon mode" to mean "boot linux straight from SPL". This
> designation makes sense, since falcons "fly at high speed and change
> direction rapidly" according to Wikipedia.
>
> The way we implement falcon mode is to reserve two areas of storage:
>    * kernel area/partition
>    * dtb area/partition
> By using some "special cases", and "spl export", SPL can more or less
> figure out how to skip u-boot.
>
>
> ## The plot twist
>
> People familiar with FIT, will have recognized that the above is
> achievable with a very basic FIT image. With some advantages:
>
>      - No "special cases" in SPL code
>      - Signed kernel images
>      - Signed kernel devicetree
>      - Devicetree overlays
>      - Automatic selection of correct devicetree
>
>
> ## The problems
>
> The advantages of FIT are not obvious by looking at SPL code. A
> noticeable amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT,
> leading one to believe that SPL_OS_BOOT is very important. It must be
> since it takes up so much code.
>
> Enabling falcon mode is not well documented, and requires a lot of trial
> and error. I've had to define 7 macros, and one new function to get it
> working on my board -- and vividly remember the grief. This is an
> antiquated way of doing things, and completely ignores the u-boot
> devicetree -- we could just as well have defined those seven values in
> the dtb.
>
> SPL assumes that it must load u-boot, unless in instances under
> CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and
> confusion over the past several months. I have no less than three patch
> series trying to address shortfalls there. It's awful.
>
>
> ## The proposal
>
> I propose we drop falcon mode support for legacy images.
>
>    - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
>    - Drop the "dtb area/partition". The dtb is derived from the FIT
>    - Drop "spl export"
>
>
> How do we deal with devicetrees in the FIT then? The options are to use
> a modified devicetree which has the desired "/chosen" node, or use DTB
> overlays.
>
> What are the reasons why we shouldn't go this route?

I think this makes sense, on the basis that it is a legacy image and
people can always use the U-Boot path if needed.

I believe Falcon boot made a lot more sense when the cache was off.
But at least in my experience, we were able to get through two SPLs
and two U-Boots and boot a kernel about 800ms on a Cortex-A15, so
Falcon mode might not be so relevant anymore, and supporting a legacy
image seems like unnecessary complexity.

Regards,
Simon

Getting rid of falcon mode

[Tom Rini]

On Wed, Apr 14, 2021 at 08:37:07PM +0100, Simon Glass wrote:
> Hi,
> 
> On Tue, 13 Apr 2021 at 09:32, Alex G. <mr.nuke.me@gmail.com> wrote:
> >
> > ## Introduction
> >
> > Today we use "falcon mode" to mean "boot linux straight from SPL". This
> > designation makes sense, since falcons "fly at high speed and change
> > direction rapidly" according to Wikipedia.
> >
> > The way we implement falcon mode is to reserve two areas of storage:
> >    * kernel area/partition
> >    * dtb area/partition
> > By using some "special cases", and "spl export", SPL can more or less
> > figure out how to skip u-boot.
> >
> >
> > ## The plot twist
> >
> > People familiar with FIT, will have recognized that the above is
> > achievable with a very basic FIT image. With some advantages:
> >
> >      - No "special cases" in SPL code
> >      - Signed kernel images
> >      - Signed kernel devicetree
> >      - Devicetree overlays
> >      - Automatic selection of correct devicetree
> >
> >
> > ## The problems
> >
> > The advantages of FIT are not obvious by looking at SPL code. A
> > noticeable amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT,
> > leading one to believe that SPL_OS_BOOT is very important. It must be
> > since it takes up so much code.
> >
> > Enabling falcon mode is not well documented, and requires a lot of trial
> > and error. I've had to define 7 macros, and one new function to get it
> > working on my board -- and vividly remember the grief. This is an
> > antiquated way of doing things, and completely ignores the u-boot
> > devicetree -- we could just as well have defined those seven values in
> > the dtb.
> >
> > SPL assumes that it must load u-boot, unless in instances under
> > CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and
> > confusion over the past several months. I have no less than three patch
> > series trying to address shortfalls there. It's awful.
> >
> >
> > ## The proposal
> >
> > I propose we drop falcon mode support for legacy images.
> >
> >    - Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
> >    - Drop the "dtb area/partition". The dtb is derived from the FIT
> >    - Drop "spl export"
> >
> >
> > How do we deal with devicetrees in the FIT then? The options are to use
> > a modified devicetree which has the desired "/chosen" node, or use DTB
> > overlays.
> >
> > What are the reasons why we shouldn't go this route?
> 
> I think this makes sense, on the basis that it is a legacy image and
> people can always use the U-Boot path if needed.
> 
> I believe Falcon boot made a lot more sense when the cache was off.
> But at least in my experience, we were able to get through two SPLs
> and two U-Boots and boot a kernel about 800ms on a Cortex-A15, so
> Falcon mode might not be so relevant anymore, and supporting a legacy
> image seems like unnecessary complexity.

I'm curious where the end of that 800ms is, and I think we might need to
post grabserial logs so everyone is on the same page about where / when
we're at in booting.

-- 
Tom
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 659 bytes
Desc: not available
URL: <https://lists.denx.de/pipermail/u-boot/attachments/20210414/17369cba/attachment.sig>