mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-23 17:30:02 +00:00
![Frank Rowand](/assets/img/avatar_default.png)
Fix various kfree() issues related to of_overlay_apply(). - Double kfree() of fdt and tree when init_overlay_changeset() returns an error. - free_overlay_changeset() free the root of the unflattened overlay (variable tree) instead of the memory that contains the unflattened overlay. - For the case of a failure during applying an overlay, move kfree() of new_fdt and overlay_mem into free_overlay_changeset(), which is called by the function that allocated them. - For the case of removing an overlay, the kfree() of new_fdt and overlay_mem remains in free_overlay_changeset(). - Check return value of of_fdt_unflatten_tree() for error instead of checking the returned value of overlay_root. - When storing pointers to allocated objects in ovcs, do so as near to the allocation as possible instead of in deeply layered function. More clearly document policy related to lifetime of pointers into overlay memory. Double kfree() Reported-by: Slawomir Stepien <slawomir.stepien@nokia.com> Signed-off-by: Frank Rowand <frank.rowand@sony.com> Signed-off-by: Rob Herring <robh@kernel.org> Link: https://lore.kernel.org/r/20220420222505.928492-3-frowand.list@gmail.com
150 lines
5.2 KiB
ReStructuredText
150 lines
5.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
========================
|
|
Devicetree 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/dynamic-resolution-notes.rst[1]
|
|
|
|
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
|
|
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::
|
|
|
|
---- foo.dts ---------------------------------------------------------------
|
|
/* FOO platform */
|
|
/dts-v1/;
|
|
/ {
|
|
compatible = "corp,foo";
|
|
|
|
/* shared resources */
|
|
res: res {
|
|
};
|
|
|
|
/* On chip peripherals */
|
|
ocp: ocp {
|
|
/* peripherals that are always instantiated */
|
|
peripheral1 { ... };
|
|
};
|
|
};
|
|
---- foo.dts ---------------------------------------------------------------
|
|
|
|
The overlay bar.dts,
|
|
::
|
|
|
|
---- bar.dts - overlay target location by label ----------------------------
|
|
/dts-v1/;
|
|
/plugin/;
|
|
&ocp {
|
|
/* bar peripheral */
|
|
bar {
|
|
compatible = "corp,bar";
|
|
... /* various properties and child nodes */
|
|
};
|
|
};
|
|
---- bar.dts ---------------------------------------------------------------
|
|
|
|
when loaded (and resolved as described in [1]) should 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 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.
|
|
|
|
If the base DT was not compiled with the -@ option then the "&ocp" label
|
|
will not be available to resolve the overlay node(s) to the proper location
|
|
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::
|
|
|
|
---- bar.dts - overlay target location by explicit path --------------------
|
|
/dts-v1/;
|
|
/plugin/;
|
|
&{/ocp} {
|
|
/* bar peripheral */
|
|
bar {
|
|
compatible = "corp,bar";
|
|
... /* various properties and child nodes */
|
|
}
|
|
};
|
|
---- bar.dts ---------------------------------------------------------------
|
|
|
|
|
|
Overlay in-kernel API
|
|
--------------------------------
|
|
|
|
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
|
|
previously created via the call to of_overlay_fdt_apply(). Removal of an
|
|
overlay changeset that is stacked by another will not be permitted.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|