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From:   Simon Glass <sjg@chromium.org>
Date:   Thu, 7 Dec 2023 13:52:53 -0700
Message-ID: <CAPnjgZ1q3yBeyYfjLumJ03HR7JM1xwN7sMbNJqfUWpsVxFTs4Q@mail.gmail.com>
Subject: Re: [PATCH v9 2/2] arm64: boot: Support Flat Image Tree
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Chen-Yu Tsai <wenst@chromium.org>,
        linux-arm-kernel@lists.infradead.org,
        Masahiro Yamada <masahiroy@kernel.org>,
        Ahmad Fatoum <a.fatoum@pengutronix.de>,
        U-Boot Mailing List <u-boot@lists.denx.de>,
        Nicolas Schier <nicolas@fjasle.eu>,
        Tom Rini <trini@konsulko.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Terrell <terrelln@fb.com>, Will Deacon <will@kernel.org>,
        linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org, workflows@vger.kernel.org
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Hi,

On Thu, 7 Dec 2023 at 07:38, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> On Thu, Dec 07, 2023 at 10:27:23PM +0800, Chen-Yu Tsai wrote:
> > On Sun, Dec 03, 2023 at 05:34:01PM +0200, Laurent Pinchart wrote:
> > > Hi Simon,
> > >
> > > Thank you for the patch.
> > >
> > > On Fri, Dec 01, 2023 at 08:54:42PM -0700, Simon Glass wrote:
> > > > Add a script which produces a Flat Image Tree (FIT), a single file
> > > > containing the built kernel and associated devicetree files.
> > > > Compression defaults to gzip which gives a good balance of size and
> > > > performance.
> > > >
> > > > The files compress from about 86MB to 24MB using this approach.
> > > >
> > > > The FIT can be used by bootloaders which support it, such as U-Boot
> > > > and Linuxboot. It permits automatic selection of the correct
> > > > devicetree, matching the compatible string of the running board with
> > > > the closest compatible string in the FIT. There is no need for
> > > > filenames or other workarounds.
> > > >
> > > > Add a 'make image.fit' build target for arm64, as well. Use
> > > > FIT_COMPRESSION to select a different algorithm.
> > > >
> > > > The FIT can be examined using 'dumpimage -l'.
> > > >
> > > > This features requires pylibfdt (use 'pip install libfdt'). It also
> > > > requires compression utilities for the algorithm being used. Supported
> > > > compression options are the same as the Image.xxx files. For now there
> > > > is no way to change the compression other than by editing the rule for
> > > > $(obj)/image.fit
> > > >
> > > > While FIT supports a ramdisk / initrd, no attempt is made to support
> > > > this here, since it must be built separately from the Linux build.
> > >
> > > FIT images are very useful, so I think this is a very welcome addition
> > > to the kernel build system. It can get tricky though: given the
> > > versatile nature of FIT images, there can't be any
> > > one-size-fits-them-all solution to build them, and striking the right
> > > balance between what makes sense for the kernel and the features that
> > > users may request will probably lead to bikeshedding. As we all love
> > > bikeshedding, I thought I would start selfishly, with a personal use
> > > case :-) This isn't a yak-shaving request though, I don't see any reason
> > > to delay merging this series.
> > >
> > > Have you envisioned building FIT images with a subset of DTBs, or adding
> > > DTBOs ? Both would be fairly trivial extensions to this script by
> > > extending the supported command line arguments. It would perhaps be more
> > > difficult to integrate in the kernel build system though. This leads me
> > > to a second question: would you consider merging extensions to this
> > > script if they are not used by the kernel build system, but meant for
> > > users who manually invoke the script ? More generally, is the script
> >
> > We'd also be interested in some customization, though in a different way.
> > We imagine having a rule file that says X compatible string should map
> > to A base DTB, plus B and C DTBO for the configuration section. The base
> > DTB would carry all common elements of some device, while the DTBOs
> > carry all the possible second source components, like different display
> > panels or MIPI cameras for instance. This could drastically reduce the
> > size of FIT images in ChromeOS by deduplicating all the common stuff.
>
> Do you envision the "mapping" compatible string mapping to a config
> section in the FIT image, that would bundle the base DTB and the DTBOs ?
>
> > > meant to be used stand-alone as well, in which case its command line
> > > arguments need to remain backward-compatible, or do you see it as being
> > > internal to the kernel ?

It is great to see all this discussion! I did send a proposal to the
U-Boot ML about extensions but it was mixed up with other things, so
I'll start a new thread.

For now, I am really just waiting for this to be applied, before
talking too much about future possibilities.

Regards,
SImon