Hi, some bikeshedding incoming. On 23.06.21 07:16, Sascha Hauer wrote: > Document the recently gained device tree overlay features. > > Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> > --- > Documentation/user/devicetree.rst | 37 +++++++++++++++++++++++++++++++ > 1 file changed, 37 insertions(+) > > diff --git a/Documentation/user/devicetree.rst b/Documentation/user/devicetree.rst > index 4be6a1e703..d78394b7aa 100644 > --- a/Documentation/user/devicetree.rst > +++ b/Documentation/user/devicetree.rst > @@ -75,3 +75,40 @@ It is important to know that these commands normally work on the internal > devicetree. If you want to modify the devicetree the kernel is started with > see the -f options to of_property and of_node. This option will register the > operation for later execution on the Kernel devicetree. > + > +Device tree overlays > +-------------------- > + > +barebox has support for device tree overlays. barebox knows two different trees, > +the live tree and the device tree the kernel is started with. Both can be applied > +overlays to. > + > +Device tree overlays on the live tree > +..................................... > + > +While the live tree can be patched by board code, barebox does not > +detect any changes to the live tree. To let the overlays have any effect, board > +code must make sure the live tree is patched before the devices are instanciated > +from it. > + > +Device tree overlays on the kernel device tree > +.............................................. > + > +Overlays can be applied to the kernel device tree before it is handed over to > +the kernel. The behaviour is controlled by different variables: > + > +``global.of_overlay_dir`` Convention seems to be that attributes of the same thing are preceded by a `.', e.g.: global.of.overlay.dir global.of.overlay.compatible global.of.overlay.filepattern (I think the `.' between of and overlay is not a bad idea, as we may decide to have e.g. global.of.compatible in future) > + Overlays are read from this directory. barebox will try to apply all overlays > + found here if not limited by one of the other variables below. When using > + bootloader spec this variable will be temporarily overwritten with > + ``$mountpoint/overlays`` where ``$mountpoint`` is the path where the filesystem > + containing the bootloader spec entries are located. Couldn't paths be interpreted relative to the $mountpoint? That way you can make overlays the default always and have the user override it if they prefer something else? > +``global.of_overlay_compatible`` > + This is a space separated list of compatibles. Only overlays matching one of > + these compatibles will be applied. When this list is empty then all overlays > + will be applied. Overlays that don't have a compatible are considered being > + always compatible. Makes sense. > +``global.of_overlay_filepattern`` > + This is a space separated list of file patterns. An overlay is only applied > + when its filename matches one of the patterns. The patterns can contain > + ``*`` and ``?`` as wildcards. When empty, all files are applied. Similarly, we could have ``*`` as a default. In future, I think we will want to have $global.of.overlay.filters. Board code can register filters that do more than look into compatibles and file patterns, e.g. compare a property with a value in EEPROM. If a user wants to make use of it, they can add the name of the filter into $global.of.overlay.filters. But this can be retrofitted: The variable will have filepattern and compatible filters as default, so no breakage. Cheers, Ahmad -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox
