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From:   Simon Glass <sjg@chromium.org>
Date:   Mon, 4 Dec 2023 10:59:44 -0700
Message-ID: <CAPnjgZ0XWDO90cq_h6V_EoOX+kmGLBRZwgT5RURCwQBQFONDYg@mail.gmail.com>
Subject: Re: [PATCH v9 2/2] arm64: boot: Support Flat Image Tree
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     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 Laurent,

On Sun, 3 Dec 2023 at 08:33, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> 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.

OK, sounds good!

>
> 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
> 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 ?

The script as written is internal to the kernel, but I am sure it
could be expanded in some ways. I am waiting to see it merged before
worrying too much about what might happen in the future!

[..]

Regards,
Simon