Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:8 as permitted sender) client-ip=2620:137:e000::3:8;
Message-ID: <8fbc81b1-31ab-46b0-87f4-b8bd8e8e2b47@pengutronix.de>
Date:   Wed, 29 Nov 2023 20:53:52 +0100
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v7 2/2] arm64: boot: Support Flat Image Tree
Content-Language: en-US
To:     Simon Glass <sjg@chromium.org>
Cc:     linux-arm-kernel@lists.infradead.org,
        Masahiro Yamada <masahiroy@kernel.org>,
        Tom Rini <trini@konsulko.com>,
        lkml <linux-kernel@vger.kernel.org>,
        U-Boot Mailing List <u-boot@lists.denx.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Terrell <terrelln@fb.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        Will Deacon <will@kernel.org>, linux-kbuild@vger.kernel.org,
        Pengutronix Kernel Team <kernel@pengutronix.de>
References: <20231129172200.430674-1-sjg@chromium.org>
 <20231129172200.430674-3-sjg@chromium.org>
 <30f32467-51ea-47de-a272-38e074f4060b@pengutronix.de>
 <CAPnjgZ25xoXsi74XYY0E8ucQiowQqPdZgUHrfVNAYWKZEYODHg@mail.gmail.com>
 <875f0dbc-1d78-4901-91b2-6ad152bcea5a@pengutronix.de>
 <CAPnjgZ0UCkm5QCx+JXe1ggSshozPnOLB6cN=UoJyMn6S4wfFkg@mail.gmail.com>
 <06281f7c-e0f2-405c-95a9-0f7e9e84a7f6@pengutronix.de>
 <CAPnjgZ2TT+0BvbjwfnGLQ3EPEXnLW6f1epiFdaBHRYaSAn5xsA@mail.gmail.com>
From:   Ahmad Fatoum <a.fatoum@pengutronix.de>
In-Reply-To: <CAPnjgZ2TT+0BvbjwfnGLQ3EPEXnLW6f1epiFdaBHRYaSAn5xsA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@pengutronix.de> wrote:
>>
>> On 29.11.23 20:27, Simon Glass wrote:
>>> On Wed, 29 Nov 2023 at 12:15, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
>>>> On 29.11.23 20:02, Simon Glass wrote:
>>>>> On Wed, 29 Nov 2023 at 11:59, Ahmad Fatoum <a.fatoum@pengutronix.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
> 

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |