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

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Message-ID: <8fbc81b1-31ab-46b0-87f4-b8bd8e8e2b47@pengutronix.de>
Date:   Wed, 29 Nov 2023 20:53:52 +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 29.11.23 20:44, Simon Glass wrote:
> Hi Ahmad,
> 
> 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.

>> The configuration already has a compatible entry. What extra use is the compatible
>> entry in the FDT node?
> 
> It allows seeing the compatible stringlist without having to read the
> FDT itself. I don't believe it is necessary though, so long as we are
> scanning the configurations and not the FDT nodes.

I think it's better to drop this if it has no use.

Cheers,
Ahmad

> 
> Regards,
> Simon
> 

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