Commit 7518b589 authored by Pantelis Antoniou's avatar Pantelis Antoniou Committed by Grant Likely

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: default avatarPantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: default avatarGrant Likely <grant.likely@linaro.org>
parent 801d728c
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.
......@@ -84,4 +84,11 @@ config OF_RESERVED_MEM
config OF_RESOLVE
bool
config OF_OVERLAY
bool
depends on OF
select OF_DYNAMIC
select OF_DEVICE
select OF_RESOLVE
endmenu # OF
......@@ -14,6 +14,7 @@ obj-$(CONFIG_OF_PCI_IRQ) += of_pci_irq.o
obj-$(CONFIG_OF_MTD) += of_mtd.o
obj-$(CONFIG_OF_RESERVED_MEM) += of_reserved_mem.o
obj-$(CONFIG_OF_RESOLVE) += resolver.o
obj-$(CONFIG_OF_OVERLAY) += overlay.o
CFLAGS_fdt.o = -I$(src)/../../scripts/dtc/libfdt
CFLAGS_fdt_address.o = -I$(src)/../../scripts/dtc/libfdt
This diff is collapsed.
......@@ -23,6 +23,7 @@
#include <linux/spinlock.h>
#include <linux/topology.h>
#include <linux/notifier.h>
#include <linux/list.h>
#include <asm/byteorder.h>
#include <asm/errno.h>
......@@ -957,4 +958,34 @@ static inline int of_reconfig_get_state_change(unsigned long action,
/* CONFIG_OF_RESOLVE api */
extern int of_resolve_phandles(struct device_node *tree);
/**
* Overlay support
*/
#ifdef CONFIG_OF_OVERLAY
/* ID based overlays; the API for external users */
int of_overlay_create(struct device_node *tree);
int of_overlay_destroy(int id);
int of_overlay_destroy_all(void);
#else
static inline int of_overlay_create(struct device_node *tree)
{
return -ENOTSUPP;
}
static inline int of_overlay_destroy(int id)
{
return -ENOTSUPP;
}
static inline int of_overlay_destroy_all(void)
{
return -ENOTSUPP;
}
#endif
#endif /* _LINUX_OF_H */
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