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Diffstat (limited to 'Documentation/devicetree/overlay-notes.rst')
| -rw-r--r-- | Documentation/devicetree/overlay-notes.rst | 48 |
1 files changed, 35 insertions, 13 deletions
diff --git a/Documentation/devicetree/overlay-notes.rst b/Documentation/devicetree/overlay-notes.rst index b2b8db765b8c..ba401ef850e7 100644 --- a/Documentation/devicetree/overlay-notes.rst +++ b/Documentation/devicetree/overlay-notes.rst @@ -14,11 +14,11 @@ How overlays work A Devicetree's overlay purpose is to modify the kernel's live tree, and have the modification affecting the state of the kernel in a way that is reflecting the changes. -Since the kernel mainly deals with devices, any new device node that result +Since the kernel mainly deals with devices, any new device node that results 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:: +Let's take an example where we have a foo board with the following base tree:: ---- foo.dts --------------------------------------------------------------- /* FOO platform */ @@ -38,10 +38,10 @@ Lets take an example where we have a foo board with the following base tree:: }; ---- foo.dts --------------------------------------------------------------- -The overlay bar.dts, +The overlay bar.dtso, :: - ---- bar.dts - overlay target location by label ---------------------------- + ---- bar.dtso - overlay target location by label --------------------------- /dts-v1/; /plugin/; &ocp { @@ -51,7 +51,7 @@ The overlay bar.dts, ... /* various properties and child nodes */ }; }; - ---- bar.dts --------------------------------------------------------------- + ---- bar.dtso -------------------------------------------------------------- when loaded (and resolved as described in [1]) should result in foo+bar.dts:: @@ -88,9 +88,9 @@ in the base DT. In this case, the target path can be provided. The target location by label syntax is preferred because the overlay can be applied to any base DT containing the label, no matter where the label occurs in the DT. -The above bar.dts example modified to use target path syntax is:: +The above bar.dtso example modified to use target path syntax is:: - ---- bar.dts - overlay target location by explicit path -------------------- + ---- bar.dtso - overlay target location by explicit path ------------------- /dts-v1/; /plugin/; &{/ocp} { @@ -100,7 +100,7 @@ The above bar.dts example modified to use target path syntax is:: ... /* various properties and child nodes */ } }; - ---- bar.dts --------------------------------------------------------------- + ---- bar.dtso -------------------------------------------------------------- Overlay in-kernel API @@ -111,7 +111,7 @@ The API is quite easy to use. 1) Call of_overlay_fdt_apply() to create and apply an overlay changeset. The return value is an error or a cookie identifying this overlay. -2) Call of_overlay_remove() to remove and cleanup the overlay changeset +2) Call of_overlay_remove() to remove and clean up the overlay changeset previously created via the call to of_overlay_fdt_apply(). Removal of an overlay changeset that is stacked by another will not be permitted. @@ -119,10 +119,32 @@ Finally, if you need to remove all overlays in one-go, just call of_overlay_remove_all() which will remove every single one in the correct order. -In addition, there is the option to register notifiers that get called on +There is the option to register notifiers that get called on overlay operations. See of_overlay_notifier_register/unregister and enum of_overlay_notify_action for details. -Note that a notifier callback is not supposed to store pointers to a device -tree node or its content beyond OF_OVERLAY_POST_REMOVE corresponding to the -respective node it received. +A notifier callback for OF_OVERLAY_PRE_APPLY, OF_OVERLAY_POST_APPLY, or +OF_OVERLAY_PRE_REMOVE may store pointers to a device tree node in the overlay +or its content but these pointers must not persist past the notifier callback +for OF_OVERLAY_POST_REMOVE. The memory containing the overlay will be +kfree()ed after OF_OVERLAY_POST_REMOVE notifiers are called. Note that the +memory will be kfree()ed even if the notifier for OF_OVERLAY_POST_REMOVE +returns an error. + +The changeset notifiers in drivers/of/dynamic.c are a second type of notifier +that could be triggered by applying or removing an overlay. These notifiers +are not allowed to store pointers to a device tree node in the overlay +or its content. The overlay code does not protect against such pointers +remaining active when the memory containing the overlay is freed as a result +of removing the overlay. + +Any other code that retains a pointer to the overlay nodes or data is +considered to be a bug because after removing the overlay the pointer +will refer to freed memory. + +Users of overlays must be especially aware of the overall operations that +occur on the system to ensure that other kernel code does not retain any +pointers to the overlay nodes or data. Any example of an inadvertent use +of such pointers is if a driver or subsystem module is loaded after an +overlay has been applied, and the driver or subsystem scans the entire +devicetree or a large portion of it, including the overlay nodes. |