[PATCH v6 01/25] doc: Add documentation about devicetree usage

Sat Dec 4 18:42:16 CET 2021

Hi François,

On Sat, 4 Dec 2021 at 04:06, François Ozog <francois.ozog at linaro.org> wrote:
>
> Hi Simon
>
> Le sam. 4 déc. 2021 à 02:02, Simon Glass <sjg at chromium.org> a écrit :
>>
>> Hi Heinrich,
>>
>> On Fri, 3 Dec 2021 at 13:28, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>> >
>> > On 12/3/21 9:13 PM, Simon Glass wrote:
>> > > Hi Heinrich,
>> > >
>> > > On Fri, 3 Dec 2021 at 06:09, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>> > >>
>> > >> On 12/3/21 13:34, Heinrich Schuchardt wrote:
>> > >>> On 12/2/21 16:58, Simon Glass wrote:
>> > >>>> At present some of the ideas and techniques behind devicetree in U-Boot
>> > >>>> are assumed, implied or unsaid. Add some documentation to cover how
>> > >>>> devicetree is build, how it can be modified and the rules about using
>> > >>>> the various CONFIG_OF_... options.
>> > >>>>
>> > >>>> Signed-off-by: Simon Glass <sjg at chromium.org>
>> > >>>> Reviewed-by: Marcel Ziswiler <marcel.ziswiler at toradex.com>
>> > >>>> ---
>> > >>>> This patch attracted quite a bit of discussion here:
>> > >>>>
>> > >>>> https://patchwork.ozlabs.org/project/uboot/patch/20210909201033.755713-4-sjg@chromium.org/
>> > >>>>
>> > >>>>
>> > >>>> I have not included the text suggested by François. While I agree that
>> > >>>> it would be useful to have an introduction in this space, I do not agree
>> > >>>> that we should have two devicetrees or that U-Boot should not have its
>> > >>>> own
>> > >>>> things in the devicetree, so it is not clear to me what we should
>> > >>>> actually
>> > >>>> write.
>> > >>>>
>> > >>>> The 'Devicetree Control in U-Boot' docs were recently merged and these
>> > >>>> provide some base info, for now.
>> > >>>>
>> > >>>> 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
>> > >>>>
>> > >>>> Changes in v5:
>> > >>>> - Bring into the OF_BOARD series
>> > >>>> - Rebase to master and drop mention of OF_PRIOR_STAGE, since removed
>> > >>>> - Refer to the 'control' DTB in the first paragraph
>> > >>>> - Use QEMU instead of qemu
>> > >>>>
>> > >>>> Changes in v3:
>> > >>>> - Clarify the 'bug' refered to at the top
>> > >>>> - Reword 'This means that there' paragraph to explain U-Boot-specific
>> > >>>> things
>> > >>>> - Move to doc/develop/devicetree now that OF_CONTROL is in the docs
>> > >>>>
>> > >>>> Changes in v2:
>> > >>>> - Fix typos per Sean (thank you!) and a few others
>> > >>>> - Add a 'Use of U-Boot /config node' section
>> > >>>> - Drop mention of dm-verity since that actually uses the kernel cmdline
>> > >>>> - Explain that OF_BOARD will still work after these changes (in
>> > >>>>     'Once this bug is fixed...' paragraph)
>> > >>>> - Expand a bit on the reason why the 'Current situation' is bad
>> > >>>> - Clarify in a second place that Linux and U-Boot use the same devicetree
>> > >>>>     in 'To be clear, while U-Boot...'
>> > >>>> - Expand on why we should have rules for other projects in
>> > >>>>     'Devicetree in another project'
>> > >>>> - Add a comment as to why devicetree in U-Boot is not 'bad design'
>> > >>>> - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'
>> > >>>> - Rewrite 'Devicetree generated on-the-fly in another project' to cover
>> > >>>>     points raised on v1
>> > >>>> - Add 'Why does U-Boot have its nodes and properties?'
>> > >>>> - Add 'Why not have two devicetrees?'
>> > >>>>
>> > >>>>    doc/develop/devicetree/dt_update.rst | 555 +++++++++++++++++++++++++++
>> > >>>>    doc/develop/devicetree/index.rst     |   1 +
>> > >>>>    2 files changed, 556 insertions(+)
>> > >>>>    create mode 100644 doc/develop/devicetree/dt_update.rst
>> > >>>>
>> > > [..]
>> > >
>> > >>>> +
>> > >>>> +- The other project may not provide a way to support U-Boot's
>> > >>>> requirements for
>> > >>>> +  devicetree, such as the /config node. Note: On the U-Boot mailing
>> > >>>> linst, this
>> > >>>
>> > >>> Even if you remove these lines in 17/25 I would prefer not to introduce
>> > >>> typos here:
>> > >>>
>> > >>> %s/linst/list/
>> > >>>
>> > >
>> > > OK I can fix that.
>> > >
>> > > [..]
>> > >
>> > >>>> +Normally, supporting U-Boot's features is trivial, since the
>> > >>>> devicetree compiler
>> > >>>> +(dtc) can compile the source, including any U-Boot pieces. So the
>> > >>>> burden is
>> > >>>> +extremely low.
>> > >>>> +
>> > >>>> +In this case, the devicetree in the other project must track U-Boot's
>> > >>>> use of
>> > >>>> +device tree, so that it remains compatible. See `Devicetree in
>> > >>>> another project`_
>> > >>>> +for reasons why.
>> > >>>
>> > >>> Did you ever ask the QEMU community what they think about your ideas?
>> > >>> What was the reply?
>> > >>
>> > >> Looking at the thread
>> > >> https://lore.kernel.org/all/20210926183410.256484-1-sjg@chromium.org/
>> > >> the QEMU project said NAK. This matches the expectation that I expressed
>> > >> repeatedly.
>> > >>
>> > >> Why don't you mention the QEMU reply in this patch series and adjust
>> > >> your design accordingly?
>> > >
>> > > The QEMU maintainer may react when he sees a problem.
>> >
>> > Why are you unwilling to admit the problem? QEMU will never support
>> > U-Boot specific stuff.
>> >
>> > Please, develop concepts that solve U-Boot's needs within U-Boot.
>>
>> So you are saying that because QEMU wrote it's devicetree support with
>> Linux in mind, we should, what...? Spent 500ms merging devicetrees
>> before relocation? Move back to platdata? Delete driver model? Rewrite
>> U-Boot?
>
> heinrich did not said that. He said that QEMU team said it doesn’t want to deal with specifics of *any* payload, be it a Linux kernel, a hypervisor, TFA, U-Boot, Coreboot or *Boot.

Except that QEMU does deal with the Linux specifics. See the
qemu-arm.dts file in this series, which is directly taken from QEMU.
It has linux, properties and a chosen node. I wasn't even suggesting
that it deal with U-Boot specifics, just provide a way to adjust the
DT that it creates out of whole cloth.

> In that spirit, TFA made sure they can have the DT they need in the FIP.
> I add now: U-Boot when loaded by SPL in QEMU can follow the same pattern and have a FIT contain U-Boot and the control DTs it needs and deal with it. Binman should be used to assemble that image. Something along those lines…

Yes, except U-Boot cannot even boot from SPL without some DT
properties. See my patch

https://patchwork.ozlabs.org/project/uboot/patch/20211101011734.1614781-15-sjg@chromium.org/

I have been working on this for years. Trust me...

Regards,
Simon

>>
>>
>> U-Boot works quite nicely as it is. The problem is that people are
>> still coming to terms with U-Boot's right to use the devicetree. This
>> could take a few more years, I think, or it may never happen. Most
>> people don't even know how U-Boot works. We just need to be patient.
>>
>> Regards,
>> Simon
>>
>>
>> >
>> > Best regards
>> >
>> > Heinrich
>> >
>> > >
>> > > I have already clearly stated that there is no way we are have two
>> > > control DTBs. The overlay is also unworkable and unnecessary. That is
>> > > why I put so much effort into this patch, after all.
>>
>>
>> > >
>> > > So for now, people will just have to deal with what QEMU provides. I
>> > > sent a patch to resolve the problem which can be accepted at any point
>> > > if people complain enough. So far only François has offered support
>> > > for it.
>> > >
>> > > Regards,
>> > > Simon
>> > >
>> >
>
> --
> François-Frédéric Ozog | Director Business Development
> T: +33.67221.6485
> francois.ozog at linaro.org | Skype: ffozog
>