[media] V4L2: add documentation for V4L2 clock helpers and asynchronous probing

Add documentation for the V4L2 clock and V4L2 asynchronous probing APIs
to v4l2-framework.txt.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Guennadi Liakhovetski 2013-06-17 02:50:33 -03:00 committed by Mauro Carvalho Chehab
parent f687f3263e
commit c67f1a300a

View File

@ -325,8 +325,27 @@ that width, height and the media bus pixel code are equal on both source and
sink of the link. Subdev drivers are also free to use this function to
perform the checks mentioned above in addition to their own checks.
A device (bridge) driver needs to register the v4l2_subdev with the
v4l2_device:
There are currently two ways to register subdevices with the V4L2 core. The
first (traditional) possibility is to have subdevices registered by bridge
drivers. This can be done when the bridge driver has the complete information
about subdevices connected to it and knows exactly when to register them. This
is typically the case for internal subdevices, like video data processing units
within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
to SoCs, which pass information about them to bridge drivers, usually in their
platform data.
There are however also situations where subdevices have to be registered
asynchronously to bridge devices. An example of such a configuration is a Device
Tree based system where information about subdevices is made available to the
system independently from the bridge devices, e.g. when subdevices are defined
in DT as I2C device nodes. The API used in this second case is described further
below.
Using one or the other registration method only affects the probing process, the
run-time bridge-subdevice interaction is in both cases the same.
In the synchronous case a device (bridge) driver needs to register the
v4l2_subdev with the v4l2_device:
int err = v4l2_device_register_subdev(v4l2_dev, sd);
@ -393,6 +412,30 @@ controlled through GPIO pins. This distinction is only relevant when setting
up the device, but once the subdev is registered it is completely transparent.
In the asynchronous case subdevice probing can be invoked independently of the
bridge driver availability. The subdevice driver then has to verify whether all
the requirements for a successful probing are satisfied. This can include a
check for a master clock availability. If any of the conditions aren't satisfied
the driver might decide to return -EPROBE_DEFER to request further reprobing
attempts. Once all conditions are met the subdevice shall be registered using
the v4l2_async_register_subdev() function. Unregistration is performed using
the v4l2_async_unregister_subdev() call. Subdevices registered this way are
stored in a global list of subdevices, ready to be picked up by bridge drivers.
Bridge drivers in turn have to register a notifier object with an array of
subdevice descriptors that the bridge device needs for its operation. This is
performed using the v4l2_async_notifier_register() call. To unregister the
notifier the driver has to call v4l2_async_notifier_unregister(). The former of
the two functions takes two arguments: a pointer to struct v4l2_device and a
pointer to struct v4l2_async_notifier. The latter contains a pointer to an array
of pointers to subdevice descriptors of type struct v4l2_async_subdev type. The
V4L2 core will then use these descriptors to match asynchronously registered
subdevices to them. If a match is detected the .bound() notifier callback is
called. After all subdevices have been located the .complete() callback is
called. When a subdevice is removed from the system the .unbind() method is
called. All three callbacks are optional.
V4L2 sub-device userspace API
-----------------------------
@ -1065,3 +1108,29 @@ available event type is 'class base + 1'.
An example on how the V4L2 events may be used can be found in the OMAP
3 ISP driver (drivers/media/platform/omap3isp).
V4L2 clocks
-----------
Many subdevices, like camera sensors, TV decoders and encoders, need a clock
signal to be supplied by the system. Often this clock is supplied by the
respective bridge device. The Linux kernel provides a Common Clock Framework for
this purpose. However, it is not (yet) available on all architectures. Besides,
the nature of the multi-functional (clock, data + synchronisation, I2C control)
connection of subdevices to the system might impose special requirements on the
clock API usage. E.g. V4L2 has to support clock provider driver unregistration
while a subdevice driver is holding a reference to the clock. For these reasons
a V4L2 clock helper API has been developed and is provided to bridge and
subdevice drivers.
The API consists of two parts: two functions to register and unregister a V4L2
clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control
a clock object, similar to the respective generic clock API calls:
v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(),
v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide
clock operations that will be called when clock users invoke respective API
methods.
It is expected that once the CCF becomes available on all relevant
architectures this API will be removed.