linux-kernel - Re: [PATCH] scripts/make_fit: Support decomposing DTBs

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Date: Mon, 10 Jun 2024 09:16:36 -0600
From: Simon Glass <sjg@...omium.org>
To: Chen-Yu Tsai <wenst@...omium.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] scripts/make_fit: Support decomposing DTBs

Hi Chen-Yu,

On Wed, 5 Jun 2024 at 03:48, Chen-Yu Tsai <wenst@...omium.org> wrote:
>
> The kernel tree builds some "composite" DTBs, where the final DTB is the
> result of applying one or more DTB overlays on top of a base DTB with
> fdtoverlay.
>
> The FIT image specification already supports configurations having one
> base DTB and overlays applied on top. It is then up to the bootloader to
> apply said overlays and either use or pass on the final result. This
> allows the FIT image builder to reuse the same FDT images for multiple
> configurations, if such cases exist.
>
> The decomposition function depends on the kernel build system, reading
> back the .cmd files for the to-be-packaged DTB files to check for the
> fdtoverlay command being called. This will not work outside the kernel
> tree. The function is off by default to keep compatibility with possible
> existing users.
>
> To facilitate the decomposition and keep the code clean, the model and
> compatitble string extraction have been moved out of the output_dtb
> function. The FDT image description is replaced with the base file name
> of the included image.
>
> Signed-off-by: Chen-Yu Tsai <wenst@...omium.org>
> ---
> This is a feature I alluded to in my replies to Simon's original
> submission of the make_fit.py script [1].
>
> This is again made a runtime argument as not all firmware out there
> that boot FIT images support applying overlays. Like my previous
> submission for disabling compression for included FDT images, the
> bootloader found in RK3399 and MT8173 Chromebooks do not support
> applying overlays. Another case of this is U-boot shipped by development
> board vendors in binary form (without upstream) in an image or in
> SPI flash on the board that were built with OF_LIBFDT_OVERLAY=n.
> These would fail to boot FIT images with DT overlays. One such
> example is my Hummingboard Pulse. In these cases the firmware is
> either not upgradable or very hard to upgrade.
>
> I believe there is value in supporting these cases. A common script
> shipped with the kernel source that can be shared by distros means
> the distro people don't have to reimplement this in their downstream
> repos or meta-packages. For ChromeOS this means reducing the amount
> of package code we have in shell script.
>
> [1] https://lore.kernel.org/linux-kbuild/20231207142723.GA3187877@google.com/
> [2]
>
>  scripts/Makefile.lib |  1 +
>  scripts/make_fit.py  | 70 ++++++++++++++++++++++++++++++--------------
>  2 files changed, 49 insertions(+), 22 deletions(-)

This is a clever way to discover the included files. Does it need to
rely on the Linux build information, or could this information somehow
be in the .dtb files? I had expected some sort of overlay scheme in
the source, but perhaps people have given up on that?

Regards,
Simon