[PATCH v6 00/25] fdt: Make OF_BOARD a boolean option

[Andre Przywara]

On Thu, 2 Dec 2021 19:03:46 +0200
Ilias Apalodimas <ilias.apalodimas at linaro.org> wrote:

Hi,

> On Thu, 2 Dec 2021 at 18:38, Tom Rini <trini at konsulko.com> wrote:
> >
> > On Thu, Dec 02, 2021 at 05:33:53PM +0100, François Ozog wrote:  
> > > Hi Simon
> > >
> > > Le jeu. 2 déc. 2021 à 17:00, Simon Glass <sjg at chromium.org> a écrit :
> > >  
> > > > With Ilias' efforts we have dropped OF_PRIOR_STAGE and OF_HOSTFILE so
> > > > there are only three ways to obtain a devicetree:
> > > >
> > > >    - OF_SEPARATE - the normal way, where the devicetree is built and
> > > >       appended to U-Boot
> > > >    - OF_EMBED - for development purposes, the devicetree is embedded in
> > > >       the ELF file (also used for EFI)
> > > >    - OF_BOARD - the board figures it out on its own
> > > >
> > > > The last one is currently set up so that no devicetree is needed at all
> > > > in the U-Boot tree. Most boards do provide one, but some don't. Some
> > > > don't even provide instructions on how to boot on the board.
> > > >
> > > > The problems with this approach are documented in another patch in this
> > > > series: "doc: Add documentation about devicetree usage"
> > > >
> > > > In practice, OF_BOARD is not really distinct from OF_SEPARATE. Any board
> > > > can obtain its devicetree at runtime, even it is has a devicetree built
> > > > in U-Boot. This is because U-Boot may be a second-stage bootloader and its
> > > > caller may have a better idea about the hardware available in the machine.
> > > > This is the case with a few QEMU boards, for example.
> > > >
> > > > So it makes no sense to have OF_BOARD as a 'choice'. It should be an
> > > > option, available with either OF_SEPARATE or OF_EMBED.
> > > >
> > > > This series makes this change, adding various missing devicetree files
> > > > (and placeholders) to make the build work.
> > > >
> > > > Note: If board maintainers are able to add their own patch to add the
> > > > files, some patches in this series can be dropped.
> > > >
> > > > It also provides a few qemu clean-ups discovered along the way. The
> > > > qemu-riscv64_spl problem is fixed.
> > > >
> > > > [1]
> > > > https://patchwork.ozlabs.org/project/uboot/patch/20210919215111.3830278-3-sjg@chromium.org/
> > > >
> > > > Changes in v6:
> > > > - Fix description of OF_BOARD so it refers just to the current state
> > > > - Explain that the 'two devicetrees' refers to two *control* devicetrees
> > > > - Expand the commit message based on comments
> > > > - Expand the commit message based on comments  
> > >
> > > You haven’t addressed any concerns expressed on the mailing list.so I am
> > > not in favor of this new version either.
> > > If you make a version without « fake DTs » as you name them, there are good
> > > advances in the documentation and other areas that would be better in
> > > mainline….
> > > If I am the only one thinking this way and the patch can be accepted, I
> > > would love there is a warning in capital letters at the top of the DTS fake
> > > files that explains the intent of this fake DT, the possible outcomes of
> > > not using the one provided by the platform and the right way of dealing
> > > with DTs for the platform.  
> >
> > This is the part that I too am still unhappy about.  I do not want
> > reference or fake or whatever device trees in the U-Boot source tree.
> > We should be able to _remove_ the ones we have, that are not required,
> > with doc/board/...rst explaining how to get / view one.  Not adding
> > more.  
> 
> So this is a key point for me and the reason I completely disagree
> with this approach.  This proposal is working in the *exact* opposite
> direction and we'll never be able to get rid of device trees from
> U-Boot, even if at some point they move out of the kernel to a
> 'common' repo'.  I'll just repeat what I've been saying since v1.
> Personally I'd be way happier if we could figure out were the specific
> U-Boot config nodes are needed and when are they needed.  Based on
> what we figure out we could, pick up the device tree from a previous
> state bootloader and fix it up with our special nodes before we start
> using it, using internal DTS files (compiled to .dtbos or similar)
> that indeed belong in the u-boot tree.

I agree on that, I always felt like U-Boot is abusing the DT here for
its own purposes. If it needs to convey some configuration information,
it should do it separately. It can use the DTB *format* (hierarchically
organised key/value pairs), but this should not really be mingled with
the hardware information. Trusted-Firmware is using this idea: they use
the already included libfdt for parsing, but load the various config
"DTB"s separately. The "hw_config" DTB (as they call the actual DTB) is
treated separately.

Cheers,
Andre