[PATCH 16/17] Documentation: Add documentation for device tree overlays

[Ahmad Fatoum]

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 at pengutronix.de>
> ---
>  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 |