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From:   Simon Glass <sjg@chromium.org>
Date:   Thu, 30 Nov 2023 13:30:21 -0700
Message-ID: <CAPnjgZ1iyxk0bb56QR10N5aSphRYhLsw7Ly=z2i6rQCxP_AYPw@mail.gmail.com>
Subject: Re: [PATCH v7 2/2] arm64: boot: Support Flat Image Tree
To:     Ahmad Fatoum <a.fatoum@pengutronix.de>
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>
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Hi Ahmad,

On Wed, 29 Nov 2023 at 12:54, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
>
> 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.

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.

>
> >> 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.

OK. I cannot think of a use for it.

Regards,
Simon