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

Sat Dec 4 12:06:09 CET 2021

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.
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…

>
> 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
> > >
> >
>
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