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 <sakari.ailus@xxxxxxxxxxxxxxx> --- Documentation/driver-api/media/tx-rx.rst | 40 +++++++++++++++++------- 1 file changed, 28 insertions(+), 12 deletions(-) diff --git a/Documentation/driver-api/media/tx-rx.rst b/Documentation/driver-api/media/tx-rx.rst index 4ba274713a61..706e839e8cec 100644 --- a/Documentation/driver-api/media/tx-rx.rst +++ b/Documentation/driver-api/media/tx-rx.rst @@ -93,18 +93,34 @@ where 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*. - -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. - -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. +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. + +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. + +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 ^^^^^^^^^^^^^^^^^^^^^^^^ -- 2.30.2
