On Fri, Sep 10, 2021 at 06:28:36PM +0200, Heinrich Schuchardt wrote: > > > On 9/10/21 2:34 PM, Tom Rini wrote: > > On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote: > > > > > > > > > On 9/9/21 10:10 PM, 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 <s...@chromium.org> > > > > Reviewed-by: Marcel Ziswiler <marcel.ziswi...@toradex.com> > > > > --- > > > > > > > > Changes in v3: > > > > - Fix typos linst suppled receive EFL > > > > - Drop 'and' before 'self-defeating' > > > > - Reword mention of control of QEMU's devicetree generation > > > > - Add mention of dropping CONFIG_OF_BOARD > > > > - Clarify the 'Once this bug is fixed' paragraph a bit > > > > - Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot > > > > devicetree > > > > - Add a note at the top explaining that his patch covers 'now', not > > > > 'future' > > > > - Add note 'Note: Some boards use a devicetree in U-Boot which does not > > > > match' > > > > > > > > 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/index.rst | 1 + > > > > doc/develop/package/devicetree.rst | 583 > > > > +++++++++++++++++++++++++++++ > > > > doc/develop/package/index.rst | 1 + > > > > 3 files changed, 585 insertions(+) > > > > create mode 100644 doc/develop/package/devicetree.rst > > > > > > > > diff --git a/doc/develop/index.rst b/doc/develop/index.rst > > > > index 83c929babda..d5ad8f9fe53 100644 > > > > --- a/doc/develop/index.rst > > > > +++ b/doc/develop/index.rst > > > > @@ -36,6 +36,7 @@ Packaging > > > > :maxdepth: 1 > > > > > > > > package/index > > > > + package/devicetree > > > > > > > > Testing > > > > ------- > > > > diff --git a/doc/develop/package/devicetree.rst > > > > b/doc/develop/package/devicetree.rst > > > > new file mode 100644 > > > > index 00000000000..b1bd310d906 > > > > --- /dev/null > > > > +++ b/doc/develop/package/devicetree.rst > > > > @@ -0,0 +1,583 @@ > > > > +.. SPDX-License-Identifier: GPL-2.0+ > > > > + > > > > +Updating the devicetree > > > > +======================= > > > > + > > > > +Note: This documentation describes how things are today, mostly, with > > > > some > > > > +mention of things that need to be fixed. It is not intended to point > > > > the way to > > > > +what might be done in the future. That should be the subject of > > > > discussions on > > > > +the mailing list. > > > > + > > > > +U-Boot uses devicetree for runtime configuration and storing required > > > > blobs or > > > > +any other information it needs to operate. It is possible to update the > > > > +devicetree separately from actually building U-Boot. This provides a > > > > good degree > > > > +of control and flexibility for firmware that uses U-Boot in > > > > conjunction with > > > > +other project. > > > > + > > > > +There are many reasons why it is useful to modify the devicetree after > > > > building > > > > +it: > > > > + > > > > +- Configuration can be changed, e.g. which UART to use > > > > +- A serial number can be added > > > > +- Public keys can be added to allow image verification > > > > +- Console output can be changed (e.g. to select serial or vidconsole) > > > > + > > > > +This section describes how to work with devicetree to accomplish your > > > > goals. > > > > + > > > > +See also :doc:`../devicetree/control` for a basic summary of the > > > > available > > > > +features. > > > > + > > > > + > > > > +Devicetree source > > > > +----------------- > > > > + > > > > +Every board in U-Boot must include a devicetree sufficient to build > > > > and boot > > > > +that board on suitable hardware (or emulation). This is specified > > > > using the > > > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > > > + > > > > + > > > > +Current situation (August 2021) > > > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > + > > > > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to > > > > be empty, > > > > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has > > > > +unfortunately created an enormous amount of confusion and some wasted > > > > effort. > > > > +This was not intended and this bug will be fixed soon. > > > > + > > > > +Some of the problems created are: > > > > + > > > > +- It is not obvious that the devicetree is coming from another project > > > > + > > > > +- There is no way to see even a sample devicetree for these platform > > > > in U-Boot, > > > > + so it is hard to know what is going on, e.g. which devices are > > > > typically > > > > + present > > > > + > > > > +- 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 > > > > list, this > > > > + was only discovered after weeks of discussion and confusion > > > > + > > > > +- For QEMU specifically, consulting two QEMU source files is required, > > > > for which > > > > + there are no references in U-Boot documentation. The code is > > > > generating a > > > > + devicetree, with some control from command-line args, but it is not > > > > clear > > > > + how to add properties required by U-Boot. > > > > + > > > > +Specifically on the changes in U-Boot: > > > > + > > > > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which > > > > does have > > > > + an in-tree devicetree, but this feature has since been used for > > > > boards that > > > > + don't > > > > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger > > > > Broadcom > > > > + change with a tag indicating it only affected one board, so the > > > > change in > > > > + behaviour was not noticed at the time. It has since been used by > > > > RISC-V qemu > > > > + boards. > > > > + > > > > +Note: It is not clear that we actually need both of these. Possibly > > > > +`CONFIG_OF_BOARD` can be dropped. > > > > + > > > > +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will > > > > override > > > > > > What does "bug" refer to? Above you describe the current design not a bug. > > > > The bug is that we have two options to provide seemingly the same > > functionality. Is there a functional difference between CONFIG_OF_BOARD > > and CONFIG_OF_PRIOR_STAGE ?
Does this clarify your question? > > > > +(at runtime) the devicetree supplied with U-Boot, but will otherwise > > > > use > > > > +CONFIG_OF_SEPARATE for the in-tree build. So these two will become > > > > options, > > > > +moving out of the 'choice' in `dts/Kconfig`. To be clear, the > > > > devicetree in the > > > > +U-Boot tree may be largely for documentation and build-testing > > > > purposes, if at > > > > +runtime the devicetree if provided by another project. But the in-tree > > > > +devicetree is packaged with U-Boot as a fallback if it does not get > > > > one from a > > > > +prior stage at runtime. This does not create two devicetrees that need > > > > to be > > > > +merged, or anything like that. If the prior stage provides one, it is > > > > used as > > > > +is, with the one provided by U-Boot being ignored. > > > > + > > > > +This means that there is a basic devicetree build in the U-Boot tree, > > > > for > > > > +build-testing, consistency and documentation purposes, but at runtime > > > > U-Boot can > > > > +accept its devicetree from another source. > > > > > > The incoming devicetree may not contain any U-Boot specific stuff. So > > > don't you need the buildtime devicetree for all of this information at > > > runtime? E.g. you were requesting to move certificate blobs into the > > > build-time devicetree. > > > > This is wrong because (a) no, there's no functional reason the prior > > stage cannot populate / be pre-populated with what we need and (b) we're > > documenting what we have today. > > The problem is not functional but organizational. The prior boot stage > may be burnt into PROM while U-Boot is on an SD-card. > > Don't expect that on a board where you could install EDK II or U-Boot or > anything else the prior boot stage cares about U-Boot. > > How could a prior boot stage possibly know years ahead what is not even > yet supported in U-Boot when the prior boot stage is created? I don't follow you, sorry. Or perhaps, if you %s/U-Boot/Linux/ the above, what's your answer then? -- Tom
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