2. MIPI CCS camera sensor driver¶
The MIPI CCS camera sensor driver is a generic driver for MIPI CCS compliant camera sensors. It exposes three sub-devices representing the pixel array, the binner and the scaler.
As the capabilities of individual devices vary, the driver exposes interfaces based on the capabilities that exist in hardware.
2.1. Pixel Array sub-device¶
The pixel array sub-device represents the camera sensor's pixel matrix, as well
as analogue crop functionality present in many compliant devices. The analogue
crop is configured using the
V4L2_SEL_TGT_CROP on the source pad (0) of the
entity. The size of the pixel matrix can be obtained by getting the
The binner sub-device represents the binning functionality on the sensor. For
that purpose, selection target
V4L2_SEL_TGT_COMPOSE is supported on the
sink pad (0).
Additionally, if a device has no scaler or digital crop functionality, the source pad (1) expses another digital crop selection rectangle that can only crop at the end of the lines and frames.
The scaler sub-device represents the digital crop and scaling functionality of
the sensor. The V4L2 selection target
V4L2_SEL_TGT_CROP is used to
configure the digital crop on the sink pad (0) when digital crop is supported.
Scaling is configured using selection target
V4L2_SEL_TGT_COMPOSE on the
sink pad (0) as well.
Additionally, if the scaler sub-device exists, its source pad (1) exposes another digital crop selection rectangle that can only crop at the end of the lines and frames.
2.4. Digital and analogue crop¶
Digital crop functionality is referred to as cropping that effectively works by dropping some data on the floor. Analogue crop, on the other hand, means that the cropped information is never retrieved. In case of camera sensors, the analogue data is never read from the pixel matrix that are outside the configured selection rectangle that designates crop. The difference has an effect in device timing and likely also in power consumption.
2.5. Private controls¶
The MIPI CCS driver implements a number of private controls under
V4L2_CID_USER_BASE_CCS to control the MIPI CCS compliant camera sensors.
2.5.1. Analogue gain model¶
The CCS defines an analogue gain model where the gain can be calculated using the following formula:
gain = m0 * x + c0 / (m1 * x + c1)
Either m0 or c0 will be zero. The constants that are device specific, can be obtained from the following controls:
V4L2_CID_CCS_ANALOGUE_GAIN_M0 V4L2_CID_CCS_ANALOGUE_GAIN_M1 V4L2_CID_CCS_ANALOGUE_GAIN_C0 V4L2_CID_CCS_ANALOGUE_GAIN_C1
The analogue gain (
x in the formula) is controlled through
V4L2_CID_ANALOGUE_GAIN in this case.
2.5.2. Alternate analogue gain model¶
The CCS defines another analogue gain model called alternate analogue gain. In this case, the formula to calculate actual gain consists of linear and exponential parts:
gain = linear * 2 ^ exponent
exponent factors can be set using the
V4L2_CID_CCS_ANALOGUE_EXPONENTIAL_GAIN controls, respectively
2.5.3. Shading correction¶
The CCS standard supports lens shading correction. The feature can be controlled
V4L2_CID_CCS_SHADING_CORRECTION. Additionally, the luminance
correction level may be changed using
V4L2_CID_CCS_LUMINANCE_CORRECTION_LEVEL, where value 0 indicates no
correction and 128 indicates correcting the luminance in corners to 10 % less
than in the centre.
Shading correction needs to be enabled for luminance correction level to have an effect.
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