[media] move V4L2 clocks to a separate .rst file

Move the v4l2 clocks stuff from v4l2-framework to a separate
file and adds an attention that came from the v4l2-clk.h.

Note: as this is meant to be a temporary kAPI, and it is
used only by two drivers (soc_camera and em28xx), where
the first one is in deprecation process, it probably not
a worth effort to document its header.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
This commit is contained in:
Mauro Carvalho Chehab 2016-07-22 09:38:58 -03:00
parent 46f74f1d3e
commit b67b81d750
3 changed files with 30 additions and 25 deletions

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@ -0,0 +1,29 @@
V4L2 clocks
-----------
.. attention::
This is a temporary API and it shall be replaced by the generic
clock API, when the latter becomes widely available.
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.

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@ -9,6 +9,7 @@ Video2Linux devices
v4l2-controls
v4l2-device
v4l2-fh
v4l2-clocks
v4l2-dv-timings
v4l2-event
v4l2-flash-led-class

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@ -79,28 +79,3 @@ and the v4l2_fh struct keeps track of filehandle instances.
The V4L2 framework also optionally integrates with the media framework. If a
driver sets the struct v4l2_device mdev field, sub-devices and video nodes
will automatically appear in the media framework as entities.
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.