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authorSakari Ailus <sakari.ailus@linux.intel.com>2021-06-23 14:38:31 +0200
committerMauro Carvalho Chehab <mchehab+huawei@kernel.org>2021-08-04 14:43:50 +0200
commit013c35b22e6216b0bde04a1fc3a1a4b599a4e13f (patch)
treef268fe89a8188bbef404bae22b92b0b2f08e4776
parente5a466d4bcf962daa8fea589b09a5675bda275a1 (diff)
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 <sakari.ailus@linux.intel.com> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
-rw-r--r--Documentation/driver-api/media/tx-rx.rst40
1 files changed, 28 insertions, 12 deletions
diff --git a/Documentation/driver-api/media/tx-rx.rst b/Documentation/driver-api/media/tx-rx.rst
index 12d492d25df2..e1e9258dd862 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
^^^^^^^^^^^^^^^^^^^^^^^^

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