media: Documentation: v4l: Rework LP-11 documentation, add callbacks

Rework LP-11 and LP-111 mode documentation to make it more understandable
and useful. This involves adding pre_streamon and post_streamon callbacks
that make it possible to explicitly transition the transmitter to either
mode.

Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/driver-api/media/tx-rx.rst

···
 LP-11 and LP-111 modes
 ^^^^^^^^^^^^^^^^^^^^^^
 
-The transmitter drivers must, if possible, configure the CSI-2 transmitter to
-*LP-11 or LP-111 mode* whenever the transmitter is powered on but not active,
-and maintain *LP-11 or LP-111 mode* until stream on. Only at stream on time
-should the transmitter activate the clock on the clock lane and transition to
-*HS mode*.
+As part of transitioning to high speed mode, a CSI-2 transmitter typically
+briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period
+may be as short as 100 µs, during which the receiver observes this state and
+proceeds its own part of high speed mode transition.
 
-Some transmitters do this automatically but some have to be explicitly
-programmed to do so, and some are unable to do so altogether due to
-hardware constraints.
+Most receivers are capable of autonomously handling this once the software has
+configured them to do so, but there are receivers which require software
+involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit
+in software, especially when there is no interrupt telling something is
+happening.
 
-The receiver thus need to be configured to expect LP-11 or LP-111 mode from the
-transmitter before the transmitter driver's ``.s_stream()`` op is called.
+One way to address this is to configure the transmitter side explicitly to LP-11
+or LP-111 mode, which requires support from the transmitter hardware. This is
+not universally available. Many devices return to this state once streaming is
+stopped while the state after power-on is LP-00 or LP-000.
+
+The ``.pre_streamon()`` callback may be used to prepare a transmitter for
+transitioning to streaming state, but not yet start streaming. Similarly, the
+``.post_streamoff()`` callback is used to undo what was done by the
+``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required
+to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``.
+
+In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition
+the transmitter to the LP-11 or LP-111 mode. This also requires powering on the
+device, so this should be only done when it is needed.
+
+Receiver drivers that do not need explicit LP-11 or LP-111 mode setup are waived
+from calling the two callbacks.
 
 Stopping the transmitter
 ^^^^^^^^^^^^^^^^^^^^^^^^