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To:     Doug Anderson <dianders@chromium.org>,
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From:   Ahmad Fatoum <a.fatoum@pengutronix.de>
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Hello,

On 04.12.23 18:52, Doug Anderson wrote:> On Sat, Dec 2, 2023 at 8:37 AM Simon Glass <sjg@chromium.org> wrote:
>> On Thu, 30 Nov 2023 at 19:04, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
>>> On 30.11.23 21:30, Simon Glass wrote:
>>>> On Wed, 29 Nov 2023 at 12:54, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
>>>>> On 29.11.23 20:44, Simon Glass wrote:
>>>> I don't have an example to hand, but this is the required mechanism of
>>>> FIT. This feature has been in place for many years and is used by
>>>> ChromeOS, at least.
>>>
>>> I see the utility of a FIT configuration with
>>>
>>>     compatible = "vendor,board-rev-a", "vendor,board-rev-b";
>>>
>>> I fail to see a utility for a configuration with
>>>
>>>     compatible = "vendor,board", "vendor,SoM", "vendor,SoC";
>>>
>>> Any configuration that ends up being booted because "vendor,SoC" was matched is
>>> most likely doomed to fail. Therefore, I would suggest that only the top level
>>> configuration is written into the FIT configurations automatically.
>>
>> Firstly, I am not an expert on this.
>>
>> Say you have a board with variants. The compatible string in U-Boot
>> may be something like:
>>
>> "google,veyron-brain-rev1", "google,veyron-brain", "google,veyron",
>> "rockchip,rk3288";
>>
>> If you then have several FIT configurations, they may be something like:
>>
>> "google,veyron-brain-rev0", "google,veyron-brain", "google,veyron",
>> "rockchip,rk3288";
>> "google,veyron-brain-rev1", "google,veyron-brain", "google,veyron",
>> "rockchip,rk3288";
>> "google,veyron-brain-rev2", "google,veyron-brain", "google,veyron",
>> "rockchip,rk3288";
>>
>> You want to choose the second one, since it is a better match than the others.

Now imagine, you are building a kernel that has no DT support for the Veyron,
but instead has support for the Phytec RK3288 PCM-947:

  phytec,rk3288-pcm-947", "phytec,rk3288-phycore-som", "rockchip,rk3288

As far as I understand U-Boot code, A veyron U-Boot would boot the Phytec DT
if CONFIG_FIT_BEST_MATCH is set, although it's a bad match and a boot failure
should rather have occurred.

>> +Doug Anderson who knows a lot more about this than me.
> 
> Hopefully this is all explained by:
> 
> https://docs.kernel.org/arch/arm/google/chromebook-boot-flow.html

Thanks Doug, this was helpful. The missing information to me was that
depthcharge only compares the top-level board compatible in addition
to runtime generated board compatibles, unlike what U-Boot seems to do.

barebox only compares its top-level compatible against the FIT configuration
compatibles, so adding a full compatible list to the configuration nodes as done
by this series should be ok there as well. I think U-Boot could run into
issues though as described above.

Out of curiosity: I only heard about Depthcharge before as exploitation toolkit
for U-Boot. Can you point me at some documentation on what the Depthcharge bootloader
does what U-Boot (which was presumably used before?) doesn't?

Thanks,
Ahmad

> 

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