linux-kernel - Re: [PATCH v7 2/2] arm64: boot: Support Flat Image Tree

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Date:   Fri, 1 Dec 2023 03:03:58 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Simon Glass <sjg@...omium.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Tom Rini <trini@...sulko.com>,
        lkml <linux-kernel@...r.kernel.org>,
        U-Boot Mailing List <u-boot@...ts.denx.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Terrell <terrelln@...com>,
        Nicolas Schier <nicolas@...sle.eu>,
        Will Deacon <will@...nel.org>, linux-kbuild@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Re: [PATCH v7 2/2] arm64: boot: Support Flat Image Tree

Hello Simon,

On 30.11.23 21:30, Simon Glass wrote:
> On Wed, 29 Nov 2023 at 12:54, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>> On 29.11.23 20:44, Simon Glass wrote:
>>> On Wed, 29 Nov 2023 at 12:33, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>>>>
>>>> On 29.11.23 20:27, Simon Glass wrote:
>>>>> On Wed, 29 Nov 2023 at 12:15, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>>>>>> On 29.11.23 20:02, Simon Glass wrote:
>>>>>>> On Wed, 29 Nov 2023 at 11:59, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>>>>>>>> The specification says that this is the root U-Boot compatible,
>>>>>>>> which I presume to mean the top-level compatible, which makes sense to me.
>>>>>>>>
>>>>>>>> The code here though adds all compatible strings from the device tree though,
>>>>>>>> is this intended?
>>>>>>>
>>>>>>> Yes, since it saves needing to read in each DT just to get the
>>>>>>> compatible stringlist.
>>>>>>
>>>>>> The spec reads as if only one string (root) is supposed to be in the list.
>>>>>> The script adds all compatibles though. This is not really useful as a bootloader
>>>>>> that's compatible with e.g. fsl,imx8mm would just take the first device tree
>>>>>> with that SoC, which is most likely to be wrong. It would be better to just
>>>>>> specify the top-level compatible, so the bootloader fails instead of taking
>>>>>> the first DT it finds.
>>>>>
>>>>> We do need to have a list, since we have to support different board revs, etc.
>>>>
>>>> Can you give me an example? The way I see it, a bootloader with
>>>> compatible "vendor,board" and a FIT with configuration with compatibles:
>>>>
>>>>   "vendor,board-rev-a", "vendor,board"
>>>>   "vendor,board-rev-b", "vendor,board"
>>>>
>>>> would just result in the bootloader booting the first configuration, even if
>>>> the device is actually rev-b.
>>>
>>> You need to find the best match, not just any match. This is
>>> documented in the function comment for fit_conf_find_compat().
>>
>> In my above example, both configuration are equally good.
>> Can you give me an example where it makes sense to have multiple
>> compatibles automatically extracted from the device tree compatible?
>>
>> The way I see it having more than one compatible here just has
>> downsides.
> 
> 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.

Cheers,
Ahmad

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