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

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Date:   Wed, 29 Nov 2023 12:44:25 -0700
From:   Simon Glass <sjg@...omium.org>
To:     Ahmad Fatoum <a.fatoum@...gutronix.de>
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

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().

>
>
> >>>>> +        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?

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.

Regards,
Simon