media: v4l2-ctrls: Add average QP control

Add a control V4L2_CID_MPEG_VIDEO_AVERAGE_QP to report the average QP
value of the current encoded frame. The value applies to the last
dequeued capture buffer.
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

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

@@ -1653,6 +1653,20 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
     Quantization parameter for a P frame for FWHT. Valid range: from 1
     to 31.
 
+``V4L2_CID_MPEG_VIDEO_AVERAGE_QP (integer)``
+    This read-only control returns the average QP value of the currently
+    encoded frame. The value applies to the last dequeued capture buffer
+    (VIDIOC_DQBUF). Its valid range depends on the encoding format and parameters.
+    For H264, its valid range is from 0 to 51.
+    For HEVC, its valid range is from 0 to 51 for 8 bit and
+    from 0 to 63 for 10 bit.
+    For H263 and MPEG4, its valid range is from 1 to 31.
+    For VP8, its valid range is from 0 to 127.
+    For VP9, its valid range is from 0 to 255.
+    If the codec's MIN_QP and MAX_QP are set, then the QP will meet both requirements.
+    Codecs need to always use the specified range, rather then a HW custom range.
+    Applicable to encoders
+
 .. raw:: latex
 
     \normalsize

drivers/media/v4l2-core/v4l2-ctrls-defs.c

@@ -970,6 +970,7 @@ const char *v4l2_ctrl_get_name(u32 id)
 	case V4L2_CID_MPEG_VIDEO_LTR_COUNT:			return "LTR Count";
 	case V4L2_CID_MPEG_VIDEO_FRAME_LTR_INDEX:		return "Frame LTR Index";
 	case V4L2_CID_MPEG_VIDEO_USE_LTR_FRAMES:		return "Use LTR Frames";
+	case V4L2_CID_MPEG_VIDEO_AVERAGE_QP:			return "Average QP Value";
 	case V4L2_CID_FWHT_I_FRAME_QP:				return "FWHT I-Frame QP Value";
 	case V4L2_CID_FWHT_P_FRAME_QP:				return "FWHT P-Frame QP Value";
 
@@ -1507,6 +1508,10 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum v4l2_ctrl_type *type,
 		*max = 0xffffffffffffLL;
 		*step = 1;
 		break;
+	case V4L2_CID_MPEG_VIDEO_AVERAGE_QP:
+		*type = V4L2_CTRL_TYPE_INTEGER;
+		*flags |= V4L2_CTRL_FLAG_READ_ONLY;
+		break;
 	case V4L2_CID_PIXEL_RATE:
 		*type = V4L2_CTRL_TYPE_INTEGER64;
 		*flags |= V4L2_CTRL_FLAG_READ_ONLY;

include/uapi/linux/v4l2-controls.h

@@ -898,6 +898,8 @@ enum v4l2_mpeg_video_av1_level {
 	V4L2_MPEG_VIDEO_AV1_LEVEL_7_3 = 23
 };
 
+#define V4L2_CID_MPEG_VIDEO_AVERAGE_QP  (V4L2_CID_CODEC_BASE + 657)
+
 /*  MPEG-class control IDs specific to the CX2341x driver as defined by V4L2 */
 #define V4L2_CID_CODEC_CX2341X_BASE				(V4L2_CTRL_CLASS_CODEC | 0x1000)
 #define V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE		(V4L2_CID_CODEC_CX2341X_BASE+0)