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

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Message-ID: <CAPnjgZ0UCkm5QCx+JXe1ggSshozPnOLB6cN=UoJyMn6S4wfFkg@mail.gmail.com>
Date:   Wed, 29 Nov 2023 12:27:38 -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 12:15, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>
> Hello Simon,
>
> On 29.11.23 20:02, Simon Glass wrote:
> > Hi Ahmad,
> >
> > On Wed, 29 Nov 2023 at 11:59, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
> >>
> >> Hi,
> >>
> >> a few more comments after decompiling the FIT image:
> >>
> >> On 29.11.23 18:21, Simon Glass wrote:
> >>> +    with fsw.add_node('kernel'):
> >>> +        fsw.property_string('description', args.name)
> >>> +        fsw.property_string('type', 'kernel_noload')
> >>
> >> The specification only says no loading done, but doesn't explain what it
> >> means for a bootloader to _not_ load an image. Looking into the U-Boot commit
> >> b9b50e89d317 ("image: Implement IH_TYPE_KERNEL_NOLOAD") that introduces this,
> >> apparently no loading means ignoring load and entry address?
> >>
> >> I presume missing load and entry is something older U-Boot versions
> >> were unhappy about? Please let me know if the semantics are as I understood,
> >> so I can prepare a barebox patch supporting it.
> >
> > Oh, see my previous email.
>
> Thanks.
>
> >
> >>
> >>> +        fsw.property_string('arch', args.arch)
> >>> +        fsw.property_string('os', args.os)
> >>> +        fsw.property_string('compression', args.compress)
> >>> +        fsw.property('data', data)
> >>> +        fsw.property_u32('load', 0)
> >>> +        fsw.property_u32('entry', 0)
> >>> +
> >>> +
> >>> +def finish_fit(fsw, entries):
> >>> +    """Finish the FIT ready for use
> >>> +
> >>> +    Writes the /configurations node and subnodes
> >>> +
> >>> +    Args:
> >>> +        fsw (libfdt.FdtSw): Object to use for writing
> >>> +        entries (list of tuple): List of configurations:
> >>> +            str: Description of model
> >>> +            str: Compatible stringlist
> >>> +    """
> >>> +    fsw.end_node()
> >>> +    seq = 0
> >>> +    with fsw.add_node('configurations'):
> >>> +        for model, compat in entries:
> >>> +            seq += 1
> >>> +            with fsw.add_node(f'conf-{seq}'):
> >>> +                fsw.property('compatible', bytes(compat))
> >>
> >> 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.

>
> >>> +        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.

Regards,
Simon