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authorPantelis Antoniou <pantelis.antoniou@konsulko.com>2014-10-28 22:35:58 +0200
committerAlex Shi <alex.shi@linaro.org>2015-12-16 15:29:16 +0800
commitbca272cdcda132c83232ccd1808bbcdd8282a02e (patch)
tree24b55cddc799aada62b23836dacaffb78ce1e376 /Documentation
parent65e65047560d16a2394b45f8040c4bce87589684 (diff)
downloadlinux-linaro-stable-bca272cdcda132c83232ccd1808bbcdd8282a02e.tar.gz
of/overlay: Introduce DT overlay support
Overlays are a method to dynamically modify part of the kernel's device tree with dynamically loaded data. Add the core functionality to parse, apply and remove an overlay changeset. The core functionality takes care of managing the overlay data format and performing the add and remove. Drivers are expected to use the overlay functionality to support custom expansion busses commonly found on consumer development boards like the BeagleBone or Raspberry Pi. The overlay code uses CONFIG_OF_DYNAMIC changesets to perform the low level work of modifying the devicetree. Documentation about internal and APIs is provided in Documentation/devicetree/overlay-notes.txt v2: - Switch from __of_node_alloc() to __of_node_dup() - Documentation fixups - Remove 2-pass processing of properties - Remove separate ov_lock; just use the DT mutex. v1: - Drop delete capability using '-' prefix. The '-' prefixed names are valid properties and nodes and there is no need for it just yet. - Do not update special properties - name & phandle ones. - Change order of node attachment, so that the special property update works. Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com> Signed-off-by: Grant Likely <grant.likely@linaro.org> (cherry picked from commit 7518b5890d8ac366faa2326ce2356ef6392ce63d) Signed-off-by: Alex Shi <alex.shi@linaro.org>
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+Device Tree Overlay Notes
+-------------------------
+
+This document describes the implementation of the in-kernel
+device tree overlay functionality residing in drivers/of/overlay.c and is a
+companion document to Documentation/devicetree/dt-object-internal.txt[1] &
+Documentation/devicetree/dynamic-resolution-notes.txt[2]
+
+How overlays work
+-----------------
+
+A Device Tree's overlay purpose is to modify the kernel's live tree, and
+have the modification affecting the state of the the kernel in a way that
+is reflecting the changes.
+Since the kernel mainly deals with devices, any new device node that result
+in an active device should have it created while if the device node is either
+disabled or removed all together, the affected device should be deregistered.
+
+Lets take an example where we have a foo board with the following base tree
+which is taken from [1].
+
+---- foo.dts -----------------------------------------------------------------
+ /* FOO platform */
+ / {
+ compatible = "corp,foo";
+
+ /* shared resources */
+ res: res {
+ };
+
+ /* On chip peripherals */
+ ocp: ocp {
+ /* peripherals that are always instantiated */
+ peripheral1 { ... };
+ }
+ };
+---- foo.dts -----------------------------------------------------------------
+
+The overlay bar.dts, when loaded (and resolved as described in [2]) should
+
+---- bar.dts -----------------------------------------------------------------
+/plugin/; /* allow undefined label references and record them */
+/ {
+ .... /* various properties for loader use; i.e. part id etc. */
+ fragment@0 {
+ target = <&ocp>;
+ __overlay__ {
+ /* bar peripheral */
+ bar {
+ compatible = "corp,bar";
+ ... /* various properties and child nodes */
+ }
+ };
+ };
+};
+---- bar.dts -----------------------------------------------------------------
+
+result in foo+bar.dts
+
+---- foo+bar.dts -------------------------------------------------------------
+ /* FOO platform + bar peripheral */
+ / {
+ compatible = "corp,foo";
+
+ /* shared resources */
+ res: res {
+ };
+
+ /* On chip peripherals */
+ ocp: ocp {
+ /* peripherals that are always instantiated */
+ peripheral1 { ... };
+
+ /* bar peripheral */
+ bar {
+ compatible = "corp,bar";
+ ... /* various properties and child nodes */
+ }
+ }
+ };
+---- foo+bar.dts -------------------------------------------------------------
+
+As a result of the the overlay, a new device node (bar) has been created
+so a bar platform device will be registered and if a matching device driver
+is loaded the device will be created as expected.
+
+Overlay in-kernel API
+--------------------------------
+
+The API is quite easy to use.
+
+1. Call of_overlay_create() to create and apply an overlay. The return value
+is a cookie identifying this overlay.
+
+2. Call of_overlay_destroy() to remove and cleanup the overlay previously
+created via the call to of_overlay_create(). Removal of an overlay that
+is stacked by another will not be permitted.
+
+Finally, if you need to remove all overlays in one-go, just call
+of_overlay_destroy_all() which will remove every single one in the correct
+order.
+
+Overlay DTS Format
+------------------
+
+The DTS of an overlay should have the following format:
+
+{
+ /* ignored properties by the overlay */
+
+ fragment@0 { /* first child node */
+
+ target=<phandle>; /* phandle target of the overlay */
+ or
+ target-path="/path"; /* target path of the overlay */
+
+ __overlay__ {
+ property-a; /* add property-a to the target */
+ node-a { /* add to an existing, or create a node-a */
+ ...
+ };
+ };
+ }
+ fragment@1 { /* second child node */
+ ...
+ };
+ /* more fragments follow */
+}
+
+Using the non-phandle based target method allows one to use a base DT which does
+not contain a __symbols__ node, i.e. it was not compiled with the -@ option.
+The __symbols__ node is only required for the target=<phandle> method, since it
+contains the information required to map from a phandle to a tree location.