[RFC v5 14/15] media: uapi: Add controls for sub-sampling configuration

Sakari Ailus
2025-02-03T09:00:01Z

Sub-sampling is a way to decrease the data rates after the pixel array by
systematically discarding some samples, either vertically or horizontally
or both. Add two controls for the purpose and document them. The
sub-sampling configuration is taken into account in the compose rectangle.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
.../userspace-api/media/drivers/camera-sensor.rst | 14 ++++++++++++--
.../userspace-api/media/v4l/ext-ctrls-camera.rst | 11 +++++++++++
drivers/media/v4l2-core/v4l2-ctrls-defs.c | 2 ++
include/uapi/linux/v4l2-controls.h | 2 ++
4 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/Documentation/userspace-api/media/drivers/camera-sensor.rst b/Documentation/userspace-api/media/drivers/camera-sensor.rst
index 59431ab0b923..9e74eb02e02a 100644
--- a/Documentation/userspace-api/media/drivers/camera-sensor.rst
+++ b/Documentation/userspace-api/media/drivers/camera-sensor.rst
@@ -120,8 +120,8 @@ values programmed by the register sequences. The default values of these
controls shall be 0 (disabled). Especially these controls shall not be inverted,
independently of the sensor's mounting rotation.

-Binning
--------
+Binning and sub-sampling
+------------------------

Binning has traditionally been configured using :ref:`the compose selection
rectangle <v4l2-selection-targets-table>`. The :ref:`V4L2_CID_BINNING
@@ -130,6 +130,16 @@ users should use it when it's available. Drivers supporting the control shall
also support the compose rectangle, albeit the rectangle may be read-only when
the control is present.

+Sub-sampling is often supported as part of a camera sensor's binning
+functionality and performed after the binning operation. Sub-sampling typically
+produces quality-wise worse results than binning. Sub-sampling factors are
+independent horizontally and vertically and they are controlled using two
+controls, :ref:`V4L2_CID_SUBSAMPLING_HORIZONTAL and
+V4L2_CID_SUBSAMPLING_VERTICAL <v4l2-cid-camera-sensor-subsampling>`. In
+sub-sampling, the image size before sub-sampling is horizontally and vertically
+divided by the respective sub-sampling factors. Drivers supporting the control shall
+also reflect the sub-sampling configuration in the compose rectangle.
+
.. _media_using_camera_sensor_drivers_embedded_data:

Embedded data
diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst
index 18b484ff5d75..577b73045bee 100644
--- a/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst
+++ b/Documentation/userspace-api/media/v4l/ext-ctrls-camera.rst
@@ -701,3 +701,14 @@ enum v4l2_scene_mode -

For instance, a value of ``0x0001000300020003`` indicates binning by 3
(horizontally) * 3/2 (vertically).
+
+.. _v4l2-cid-camera-sensor-subsampling:
+
+``V4L2_CID_SUBSAMPLING_HORIZONTAL`` and ``V4L2_CID_SUBSAMPLING_VERTICAL``
+(integer)
+
+ Horizontal and vertical subsampling factors.
+
+ Sub-sampling is used to downscale an image, horizontally and vertically, by
+ discarding a part of the image data. Typically sub-sampling produces lower
+ quality images than binning.
diff --git a/drivers/media/v4l2-core/v4l2-ctrls-defs.c b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
index 6be3d4a37127..5f4a8cc9f53b 100644
--- a/drivers/media/v4l2-core/v4l2-ctrls-defs.c
+++ b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
@@ -1088,6 +1088,8 @@ const char *v4l2_ctrl_get_name(u32 id)
case V4L2_CID_CAMERA_SENSOR_ROTATION: return "Camera Sensor Rotation";
case V4L2_CID_HDR_SENSOR_MODE: return "HDR Sensor Mode";
case V4L2_CID_BINNING_FACTORS: return "Binning Factors";
+ case V4L2_CID_SUBSAMPLING_HORIZONTAL: return "Sub-Sampling Factor, Horizontal";
+ case V4L2_CID_SUBSAMPLING_VERTICAL: return "Sub-Sampling Factor, Vertical";

/* FM Radio Modulator controls */
/* Keep the order of the 'case's the same as in v4l2-controls.h! */
diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/linux/v4l2-controls.h
index ac0e38e98a38..4eade177d40f 100644
--- a/include/uapi/linux/v4l2-controls.h
+++ b/include/uapi/linux/v4l2-controls.h
@@ -1089,6 +1089,8 @@ enum v4l2_auto_focus_range {

#define V4L2_CID_HDR_SENSOR_MODE (V4L2_CID_CAMERA_CLASS_BASE+36)
#define V4L2_CID_BINNING_FACTORS (V4L2_CID_CAMERA_CLASS_BASE+37)
+#define V4L2_CID_SUBSAMPLING_HORIZONTAL (V4L2_CID_CAMERA_CLASS_BASE+38)
+#define V4L2_CID_SUBSAMPLING_VERTICAL (V4L2_CID_CAMERA_CLASS_BASE+39)

/* FM Modulator class control IDs */

--
2.39.5