libcamera: controls: Updates to gain and exposure controls

Rename:
ManualExposure -> ExposureTime
ManualGain -> AnalogueGain

Use micro-seconds units for ExposureTime. This is changed from milli-
seconds. The latter would not allow very low exposure times.
AnalogueGain switch to use a float to allow fractional gain adjustments.

Update the uvcvideo pipeline handler to use the new exposure and gain
units. For ExposureTime, UVC uses units of 100 micro-seconds, so map
the values before setting V4L2_CID_EXPOSURE_ABSOLUTE. For AnalogueGain,
UVC has no explicit gain units, so map the default gain value to 1.0
and linearly scale to the requested value.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/control_ids.yaml

@@ -12,7 +12,7 @@ controls:
       description: |
         Enable or disable the AE.
 
-        \sa ManualExposure
+        \sa ExposureTime AnalogueGain
 
   - AeLocked:
       type: bool
@@ -30,8 +30,6 @@ controls:
       description: |
         Enable or disable the AWB.
 
-        \sa ManualGain
-
   - Brightness:
       type: int32_t
       description: Specify a fixed brightness parameter
@@ -44,12 +42,20 @@ controls:
       type: int32_t
       description: Specify a fixed saturation parameter
 
-  - ManualExposure:
+  - ExposureTime:
       type: int32_t
-      description: Specify a fixed exposure time in milli-seconds
+      description: |
+        Exposure time (shutter speed) for the frame applied in the sensor
+        device. This value is specified in micro-seconds.
 
-  - ManualGain:
-      type: int32_t
-      description: Specify a fixed gain parameter
+        \sa AnalogueGain AeEnable
 
+  - AnalogueGain:
+      type: float
+      description: |
+        Analogue gain value applied in the sensor device.
+        The value of the control specifies the gain multiplier applied to all
+        colour channels. This value cannot be lower than 1.0.
+
+        \sa ExposureTime AeEnable
 ...