Hi Bin, On Sat, 28 Aug 2021 at 07:15, Bin Meng <bmeng...@gmail.com> wrote: > > Hi Simon, > > On Sat, Aug 28, 2021 at 11:23 AM Simon Glass <s...@chromium.org> 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> > > --- > > > > doc/develop/index.rst | 1 + > > doc/develop/package/devicetree.rst | 315 +++++++++++++++++++++++++++++ > > doc/develop/package/index.rst | 1 + > > 3 files changed, 317 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..fccbb182f3e > > --- /dev/null > > +++ b/doc/develop/package/devicetree.rst > > @@ -0,0 +1,315 @@ > > +.. SPDX-License-Identifier: GPL-2.0+ > > + > > +Updating the devicetree > > +======================= > > + > > +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. Specifically: > > + > > +- `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. > > + > > +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will > > override > > +(at runtime) the devicetree suppled 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` > > + > > +Offending boards are: > > + > > +- bcm7260 > > +- bcm7445 > > +- qemu_arm64 > > +- qemu_arm > > +- qemu-ppce500 > > +- qemu-riscv32 > > +- qemu-riscv32_smode > > +- qemu-riscv64 > > +- qemu-riscv64_smode > > + > > +All of these need to have a devicetree added in-tree. This is targeted to > > be > > +fixed in the 2022.01 release. > > Do you have some idea of how to support the QEMU case, if not using > CONFIG_OF_PRIOR_STAGE?
To be clear, I am not planning to remove this option. It will still work. But I do want to have an in-tree devicetree for all boards, and someone will need to update qemu to support adding U-Boot nodes/properties. It always has an array of options controlling what it presents to BIOS/UEFI, for example, so this should not be too controversial. Regards, Simon
