[PATCH 00/16] fdt: Make OF_BOARD a boolean option

[Simon Glass]

Hi François,

On Thu, 14 Oct 2021 at 12:13, François Ozog <francois.ozog at linaro.org> wrote:
>
>
>
> Le mer. 13 oct. 2021 à 20:06, Simon Glass <sjg at chromium.org> a écrit :
>>
>> Hi François,
>>
>> On Wed, 13 Oct 2021 at 11:35, François Ozog <francois.ozog at linaro.org> wrote:
>> >
>> > Hi Simon
>> >
>> > Le mer. 13 oct. 2021 à 16:49, Simon Glass <sjg at chromium.org> a écrit :
>> >>
>> >> Hi Tom, Bin,François,
>> >>
>> >> On Tue, 12 Oct 2021 at 19:34, Tom Rini <trini at konsulko.com> wrote:
>> >> >
>> >> > On Wed, Oct 13, 2021 at 09:29:14AM +0800, Bin Meng wrote:
>> >> > > Hi Simon,
>> >> > >
>> >> > > On Wed, Oct 13, 2021 at 9:01 AM Simon Glass <sjg at chromium.org> wrote:
>> >> > > >
>> >> > > > 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 at [1].
>> >> > > >
>> >> > > > 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.
>> >> > >
>> >> > > Adding device trees that are never used sounds like a hack to me.
>> >> > >
>> >> > > For QEMU, device tree is dynamically generated on the fly based on
>> >> > > command line parameters, and the device tree you put in this series
>> >> > > has various hardcoded <phandle> values which normally do not show up
>> >> > > in hand-written dts files.
>> >> > >
>> >> > > I am not sure I understand the whole point of this.
>> >> >
>> >> > I am also confused and do not like the idea of adding device trees for
>> >> > platforms that are capable of and can / do have a device tree to give us
>> >> > at run time.
>> >>
>> >> (I'll just reply to this one email, since the same points applies to
>> >> all replies I think)
>> >>
>> >> I have been thinking about this and discussing it with people for a
>> >> few months now. I've been signalling a change like this for over a
>> >> month now, on U-Boot contributor calls and in discussions with Linaro
>> >> people. I sent a patch (below) to try to explain things. I hope it is
>> >> not a surprise!
>> >>
>> >> The issue here is that we need a devicetree in-tree in U-Boot, to
>> >> avoid the mess that has been created by OF_PRIOR_STAGE, OF_BOARD,
>> >> BINMAN_STANDALONE_FDT and to a lesser extent, OF_HOSTFILE. Between
>> >> Ilias' series and this one we can get ourselves on a stronger footing.
>> >> There is just OF_SEPARATE, with OF_EMBED for debugging/ELF use.
>> >> For more context:
>> >>
>> >> http://patchwork.ozlabs.org/project/uboot/patch/20210919215111.3830278-3-sjg@chromium.org/
>> >>
>> >> BTW I did suggest to QEMU ARM that they support a way of adding the
>> >> u-boot.dtsi but there was not much interest there (in fact the
>> >> maintainer would prefer there was no special support even for booting
>> >> Linux directly!)
>> >
>> > i understand their point of view and agree with it.
>> >>
>> >> But in any case it doesn't really help U-Boot. I
>> >> think the path forward might be to run QEMU twice, once to get its
>> >> generated tree and once to give the 'merged' tree with the U-Boot
>> >> properties in it, if people want to use U-Boot features.
>> >>
>> >> I do strongly believe that OF_BOARD must be a run-time option, not a
>> >> build-time one. It creates all sorts of problems and obscurity which
>> >> have taken months to unpick. See the above patch for the rationale.
>> >>
>> >> To add to that rationale, OF_BOARD needs to be an option available to
>> >> any board. At some point in the future it may become a common way
>> >> things are done, e.g. TF-A calling U-Boot and providing a devicetree
>> >> to it. It doesn't make any sense to have people decide whether or not
>> >> to set OF_BOARD at build time, thus affecting how the image is put
>> >> together. We'll end up with different U-Boot build targets like
>> >> capricorn, capricorn_of_board and the like. It should be obvious where
>> >> that will lead. Instead, OF_BOARD needs to become a commonly used
>> >> option, perhaps enabled by most/all boards, so that this sort of build
>> >> explosion is not needed.
>> >
>> > If you mean that when boards are by construction providing a DTB to U-Boot then I agree very much. But I don’t understand how the patch set  supports it as it puts dts files for those boards to be built.
>> >>
>> >> U-Boot needs to be flexible enough to
>> >> function correctly in whatever runtime environment in which it finds
>> >> itself.
>> >>
>> >> Also as binman is pressed into service more and more to build the
>> >> complex firmware images that are becoming fashionable, it needs a
>> >> definition (in the devicetree) that describes how to create the image.
>> >> We can't support that unless we are building a devicetree, nor can the
>> >> running program access the image layout without that information.
>> >>
>> >> François's point about 'don't use this with any kernel' is
>> >> germane...but of course I am not suggesting doing that, since OF_BOARD
>> >> is, still, enabled. We already use OF_BOARD for various boards that
>> >> include an in-tree devicetree - Raspberry Pi 1, 2 and 3, for example
>> >> (as I said in the cover letter "Most boards do provide one, but some
>> >> don't."). So this series is just completing the picture by enforcing
>> >> that *some sort* of devicetree is always present.
>> >
>> > That seems inconsistent with the OF_BOARD becomes the default.
>>
>> I think the key point that will get you closer to where I am on this
>> issue, is that OF_BOARD needs to be a run-time option. At present it
>> has build-time effects and this is quite wrong.
>
> Doesn’t that mean that the current build system is not fully supporting boards that do provide the DT and you try to hack your way in ? As I replied to Tom, I could accept temporarily a void.dts containing nothing to actually uallly pass any build problem until we properly fix the build system.
> (But no “close to real” fake dts in the dts section)

Can you rephrase that paragraph, particularly the first setence? I am
not sure what you are getting at.

>>
>> If you go through all
>> the material I have written on this I think I have motivated that very
>> clearly.
>>
>> Another big issue is that I believe we need ONE devicetree for U-Boot,
>> not two that get merged by U-Boot. Again I have gone through that in a
>> lot of detail.
>
> i know but a number of people do not agree with your position. U-Boot can leverage many DTs coming from hats and capes to finalize the DT it passes to OS. It can also leverage a file, a FIP section (NT_FW_CONFIG in TFA) a FIT section formatted as FDT for its own configuration.

Perhaps it wasn't clear from the context, but I was talking about the
devicetree for U-Boot, i.e. the control DTB for U-Boot. It's fine to
merge overlays, etc. to pass to Linux, of course.

>>
>>
>> >>
>> >>
>> >> I can't quite pinpoint the patch where U-Boot started allowing the
>> >> devicetree to be omitted, but if people are interested I could try a
>> >> little harder. It was certainly not my intention (putting on my
>> >> device-tree-maintainer hat) to go down that path and it slipped in
>> >> somehow in all the confusion. I'm not sure anyone could tell you that
>> >> rpi_3 has an in-tree devicetree but rpi_4 does not...
>> >>
>> >> Anyway this series is very modest. It just adds the requirement that
>> >> all in-tree boards have some sort of sample devicetree and preferably
>> >> some documentation as to where it might come from at runtime.
>> >
>> > That’s a very good goal. But adding files in dts make them not samples but templates to be used and replace board provided DTB.
>> > If you push all your DTS files in documentation, you do what you say: adding sample files.
>> >>
>> >> That
>> >> should not be a tough call IMO. Assuming we can get the validation in
>> >> place (mentioned by Rob Herring recently) it will be quite natural to
>> >> sync them between (presumably) Linux and U-Boot.
>> >>
>> >> I am also quite happy to discuss what should actually be in these
>> >> devicetree files and whether some of them should be essentially empty.
>> >> As you probably noticed, some of them are empty since I literally
>> >> could not figure out where they come from! But there needs to be at
>> >> least some skeleton for U-Boot to progress, since devicetree is
>> >> critical to its feature set.
>> >
>> > absolutely. And thank you for your efforts to make that center stage. This is also Linaro Edge group mist challenging  task on the next 6 moths. Knowing that we have lived in a floating situation for over 10 years, I just hope we get consensus across projects and distro providers about the right end goal and migration strategy.
>> >>
>>
>> Thank you.
>>
>> >>
>> >>
>> >> It is high time we tidied all this up. I predict it will be much
>> >> harder, and much more confusing, in 6 months.
>>
>> Just to set a road map here in case you can help unblock anything,
>> here are the things I am aware of, excluding the things I have
>> forgotten:
>>
>> - Ilias OF_PRIOR_STAGE, OF_HOSTFILE series
>> - this series
>> - the devicetree docs patch
>> - devicetree bindings upstream for U-Boot (first patch under discussion)
>> - bloblist as a means of passing devicetree, ACPI, tiny config info as
>
> the “ABI” of U-Boot entry need more specification. Having something close to Linux helped to get U-Boot in the RPI4 and other boards I believe. So we could start from here. The blob list may be an extra arg (x0=DTB, x1=bloblist in Arm).

Yes that's my intent, hopefully in a few weeks.

>>
>>
>> C structs to U-Boot (needs to be written)
>> - VPL so we can handle verification (patches pending)
>> - bootflow / VBE v2 series (coming next week)

Regards,
Simon