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From:   Doug Anderson <dianders@chromium.org>
Date:   Mon, 4 Dec 2023 09:52:56 -0800
Message-ID: <CAD=FV=WXQrbnjeVOLp--vmLzScvW9VLFRu+apSY4WnL9JFk-AQ@mail.gmail.com>
Subject: Re: [PATCH v7 2/2] arm64: boot: Support Flat Image Tree
To:     Simon Glass <sjg@chromium.org>
Cc:     Ahmad Fatoum <a.fatoum@pengutronix.de>,
        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,

On Sat, Dec 2, 2023 at 8:37=E2=80=AFAM Simon Glass <sjg@chromium.org> wrote=
:
>
> Hi Ahmad,
>
> On Thu, 30 Nov 2023 at 19:04, Ahmad Fatoum <a.fatoum@pengutronix.de> wrot=
e:
> >
> > Hello Simon,
> >
> > On 30.11.23 21:30, Simon Glass wrote:
> > > On Wed, 29 Nov 2023 at 12:54, Ahmad Fatoum <a.fatoum@pengutronix.de> =
wrote:
> > >> On 29.11.23 20:44, Simon Glass wrote:
> > >>> 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@pengutroni=
x.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 devi=
ce 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 usefu=
l 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 bet=
ter to just
> > >>>>>> specify the top-level compatible, so the bootloader fails instea=
d of taking
> > >>>>>> the first DT it finds.
> > >>>>>
> > >>>>> We do need to have a list, since we have to support different boa=
rd 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 compat=
ibles:
> > >>>>
> > >>>>   "vendor,board-rev-a", "vendor,board"
> > >>>>   "vendor,board-rev-b", "vendor,board"
> > >>>>
> > >>>> would just result in the bootloader booting the first configuratio=
n, 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 o=
f
> > > 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 =3D "vendor,board-rev-a", "vendor,board-rev-b";
> >
> > I fail to see a utility for a configuration with
> >
> >     compatible =3D "vendor,board", "vendor,SoM", "vendor,SoC";
> >
> > Any configuration that ends up being booted because "vendor,SoC" was ma=
tched is
> > most likely doomed to fail. Therefore, I would suggest that only the to=
p level
> > configuration is written into the FIT configurations automatically.
>
> Firstly, I am not an expert on this.
>
> Say you have a board with variants. The compatible string in U-Boot
> may be something like:
>
> "google,veyron-brain-rev1", "google,veyron-brain", "google,veyron",
> "rockchip,rk3288";
>
> If you then have several FIT configurations, they may be something like:
>
> "google,veyron-brain-rev0", "google,veyron-brain", "google,veyron",
> "rockchip,rk3288";
> "google,veyron-brain-rev1", "google,veyron-brain", "google,veyron",
> "rockchip,rk3288";
> "google,veyron-brain-rev2", "google,veyron-brain", "google,veyron",
> "rockchip,rk3288";
>
> You want to choose the second one, since it is a better match than the ot=
hers.
>
> +Doug Anderson who knows a lot more about this than me.

Hopefully this is all explained by:

https://docs.kernel.org/arch/arm/google/chromebook-boot-flow.html