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

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Date:   Wed, 29 Nov 2023 20:33:48 +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

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.


>>>>> +        fsw.property_string('description', model)
>>>>> +        fsw.property_string('type', 'flat_dt')
>>>>> +        fsw.property_string('arch', arch)
>>>>> +        fsw.property_string('compression', compress)
>>>>> +        fsw.property('compatible', bytes(compat))
>>>>
>>>> I think I've never seen a compatible for a fdt node before.
>>>> What use does this serve?
>>>
>>> It indicates the machine that the DT is for.
>>
>> Who makes use of this information?
> 
> U-Boot uses it, I believe. There is an optimisation to use this
> instead of reading the DT itself.

The configuration already has a compatible entry. What extra use is the compatible
entry in the FDT node?

Thanks,
Ahmad

> 
> Regards,
> Simon
> 

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