Document the suggested way to exposure controls for exposure and gain for camera sensor drivers. Signed-off-by: Jacopo Mondi <jacopo.mondi@xxxxxxxxxxxxxxxx> Reviewed-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx> --- .../driver-api/media/camera-sensor.rst | 27 +++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst index cd915ca119ea..67fe77b1edb9 100644 --- a/Documentation/driver-api/media/camera-sensor.rst +++ b/Documentation/driver-api/media/camera-sensor.rst @@ -189,3 +189,30 @@ the ``V4L2_CID_VFLIP`` and ``V4L2_CID_HFLIP`` controls with the a flip can potentially change the output buffer content layout. Flips should also be taken into account when enumerating and handling media bus formats on the camera sensor source pads. + +Exposure and Gain Control +------------------------- + +Camera sensor drivers that allow applications to control the image exposure +and gain should do so by exposing dedicated controls to applications. + +Exposure time is controlled by registering the ``V4L2_CID_EXPOSURE`` control. +The control definition does not specify a unit to allow maximum flexibility +for multiple device types, but when used for camera sensor drivers it should be +expressed in unit of lines whenever possible. + +To convert lines into units of time, the total line length (visible and +not visible pixels) has to be divided by the pixel rate:: + + line duration = total line length / pixel rate + = (image width + horizontal blanking) / pixel rate + +Camera sensor driver should try whenever possible to distinguish between the +analogue and digital gain control functions. Analogue gain is a multiplication +factor applied to all color channels on the pixel array before they get +converted into the digital domain. It should be made controllable by +registering the ``V4L2_CID_ANALOGUE_GAIN`` control, expressed as a device +specific gain code. Digital gain control is optional and should be exposed to +applications by registering ``V4L2_CID_DIGITAL_GAIN``. Camera sensor drivers are +discouraged from using ``V4L2_CID_GAIN`` as it doesn't allow differentiation of +analogue vs digital gain. -- 2.40.1
