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Diffstat (limited to 'Documentation/user/devicetree.rst')
| -rw-r--r-- | Documentation/user/devicetree.rst | 45 |
1 files changed, 44 insertions, 1 deletions
diff --git a/Documentation/user/devicetree.rst b/Documentation/user/devicetree.rst index 679cae7f00..91afffdcda 100644 --- a/Documentation/user/devicetree.rst +++ b/Documentation/user/devicetree.rst @@ -21,7 +21,7 @@ The internal devicetree ----------------------- The devicetree consulted by barebox plays a special role. It is referred to -as the "internal devicetree." The barebox devicetree commands work on this +as the "internal devicetree" or "live tree". The barebox devicetree commands work on this devicetree. The devicetree source (DTS) files are kept in sync with the kernel DTS files. As the FDT files are meant to be backward compatible, it should always be possible to start a kernel with the barebox internal devicetree. However, since the barebox @@ -75,3 +75,46 @@ 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`` + 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 the path + given here is an absolute path it is used as is. A relative path is relative + to ``/`` or relative to the rootfs when using bootloader spec. +``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. +``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. The default is ``*`` which means all files are + applied. +``global.of.overlay.filter`` + This is a space separated list of filters to apply. There are two generic filters: + ``filepattern`` matches ``global.of.overlay.filepattern`` above, ``compatible`` matches + ``global.of.overlay.compatible`` above. The default is ``filepattern compatible`` + which means the two generic filters are active. This list may be replaced or + supplemented by board specific filters. |