Re: [PATCH] scripts/make_fit: Support decomposing DTBs

[Elliot Berman <quic_eberman@quicinc.com>]

On Tue, Jun 11, 2024 at 04:51:52PM +0800, Chen-Yu Tsai wrote:
> On Mon, Jun 10, 2024 at 11:16 PM Simon Glass <sjg@chromium.org> wrote:
> >
> > Hi Chen-Yu,
> >
> > On Wed, 5 Jun 2024 at 03:48, Chen-Yu Tsai <wenst@chromium.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@chromium.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
> 
> (+CC DT folks and mailing list)
> 
> I suppose we could make the `fdtoverlay` program embed this data during
> the kernel build. That would keep the information together, while also
> having one source of truth (the kernel Makefiles). Whether it belongs
> in the DTB or not is a separate matter.
> 
> > the source, but perhaps people have given up on that?
> 
> I wouldn't say given up, since we haven't agreed on anything either.
> Elliot had some concerns when I brought this up earlier [1] though.
> 

I'd be happy if it was from the DTS.

Thanks,
Elliot