media: v4l2-ctrl: Document V4L2_CID_CAMERA_SENSOR_ROTATION

Add documentation for the V4L2_CID_CAMERA_SENSOR_ROTATION camera
control. The newly added read-only control reports the rotation
correction to be applied to images before displaying them to the user.
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst

@@ -540,6 +540,127 @@ enum v4l2_scene_mode -
 
 
 
+``V4L2_CID_CAMERA_SENSOR_ROTATION (integer)``
+    This read-only control describes the rotation correction in degrees in the
+    counter-clockwise direction to be applied to the captured images once
+    captured to memory to compensate for the camera sensor mounting rotation.
+
+    For a precise definition of the sensor mounting rotation refer to the
+    extensive description of the 'rotation' properties in the device tree
+    bindings file 'video-interfaces.txt'.
+
+    A few examples are below reported, using a shark swimming from left to
+    right in front of the user as the example scene to capture. ::
+
+                 0               X-axis
+               0 +------------------------------------->
+                 !
+                 !
+                 !
+                 !           |\____)\___
+                 !           ) _____  __`<
+                 !           |/     )/
+                 !
+                 !
+                 !
+                 V
+               Y-axis
+
+    Example one - Webcam
+
+    Assuming you can bring your laptop with you while swimming with sharks,
+    the camera module of the laptop is installed on the user facing part of a
+    laptop screen casing, and is typically used for video calls. The captured
+    images are meant to be displayed in landscape mode (width > height) on the
+    laptop screen.
+
+    The camera is typically mounted upside-down to compensate the lens optical
+    inversion effect. In this case the value of the
+    V4L2_CID_CAMERA_SENSOR_ROTATION control is 0, no rotation is required to
+    display images correctly to the user.
+
+    If the camera sensor is not mounted upside-down it is required to compensate
+    the lens optical inversion effect and the value of the
+    V4L2_CID_CAMERA_SENSOR_ROTATION control is 180 degrees, as images will
+    result rotated when captured to memory. ::
+
+                 +--------------------------------------+
+                 !                                      !
+                 !                                      !
+                 !                                      !
+                 !              __/(_____/|             !
+                 !            >.___  ____ (             !
+                 !                 \(    \|             !
+                 !                                      !
+                 !                                      !
+                 !                                      !
+                 +--------------------------------------+
+
+    A software rotation correction of 180 degrees has to be applied to correctly
+    display the image on the user screen. ::
+
+                 +--------------------------------------+
+                 !                                      !
+                 !                                      !
+                 !                                      !
+                 !             |\____)\___              !
+                 !             ) _____  __`<            !
+                 !             |/     )/                !
+                 !                                      !
+                 !                                      !
+                 !                                      !
+                 +--------------------------------------+
+
+    Example two - Phone camera
+
+    It is more handy to go and swim with sharks with only your mobile phone
+    with you and take pictures with the camera that is installed on the back
+    side of the device, facing away from the user. The captured images are meant
+    to be displayed in portrait mode (height > width) to match the device screen
+    orientation and the device usage orientation used when taking the picture.
+
+    The camera sensor is typically mounted with its pixel array longer side
+    aligned to the device longer side, upside-down mounted to compensate for
+    the lens optical inversion effect.
+
+    The images once captured to memory will be rotated and the value of the
+    V4L2_CID_CAMERA_SENSOR_ROTATION will report a 90 degree rotation. ::
+
+
+                 +-------------------------------------+
+                 |                 _ _                 |
+                 |                \   /                |
+                 |                 | |                 |
+                 |                 | |                 |
+                 |                 |  >                |
+                 |                <  |                 |
+                 |                 | |                 |
+                 |                   .                 |
+                 |                  V                  |
+                 +-------------------------------------+
+
+    A correction of 90 degrees in counter-clockwise direction has to be
+    applied to correctly display the image in portrait mode on the device
+    screen. ::
+
+                          +--------------------+
+                          |                    |
+                          |                    |
+                          |                    |
+                          |                    |
+                          |                    |
+                          |                    |
+                          |   |\____)\___      |
+                          |   ) _____  __`<    |
+                          |   |/     )/        |
+                          |                    |
+                          |                    |
+                          |                    |
+                          |                    |
+                          |                    |
+                          +--------------------+
+
+
 .. [#f1]
    This control may be changed to a menu control in the future, if more
    options are required.