[libcamera-devel] [PATCH] libcamera: v4l2_videodevice: Identify non-zero stream starts

[Kieran Bingham]

V4L2 Video devices should always start streaming from index zero.
Identify and report a warning if they don't, and correct for any offset.

Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
---

I'm not yet sure if the Warning print is perhaps just unhelpful.
It doesn't really affect us if we have this work around in - so we can
be silent, and I'm not yet sure if V4L2 has any 'requirement' that it
should start at 0 at every stream on.

 include/libcamera/internal/v4l2_videodevice.h |  1 +
 src/libcamera/v4l2_videodevice.cpp            | 15 +++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/libcamera/internal/v4l2_videodevice.h b/include/libcamera/internal/v4l2_videodevice.h
index 23f37f83f8e2..8525acbc558d 100644
--- a/include/libcamera/internal/v4l2_videodevice.h
+++ b/include/libcamera/internal/v4l2_videodevice.h
@@ -276,6 +276,7 @@ private:
 	EventNotifier *fdBufferNotifier_;
 
 	State state_;
+	std::optional<unsigned int> firstFrame_;
 
 	Timer watchdog_;
 	utils::Duration watchdogDuration_;
diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
index 430715afd554..63911339f96e 100644
--- a/src/libcamera/v4l2_videodevice.cpp
+++ b/src/libcamera/v4l2_videodevice.cpp
@@ -1771,6 +1771,19 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
 	if (V4L2_TYPE_IS_OUTPUT(buf.type))
 		return buffer;
 
+	/*
+	 * Detect kernel drivers which do not reset the sequence number to zero
+	 * on stream start.
+	 */
+	if (!firstFrame_) {
+		if (buf.sequence)
+			LOG(V4L2, Warning)
+				<< "Zero sequence expected for first frame (got "
+				<< buf.sequence << ")";
+		firstFrame_ = buf.sequence;
+	}
+	buffer->metadata_.sequence -= firstFrame_.value();
+
 	unsigned int numV4l2Planes = multiPlanar ? buf.length : 1;
 	FrameMetadata &metadata = buffer->metadata_;
 
@@ -1847,6 +1860,8 @@ int V4L2VideoDevice::streamOn()
 {
 	int ret;
 
+	firstFrame_.reset();
+
 	ret = ioctl(VIDIOC_STREAMON, &bufferType_);
 	if (ret < 0) {
 		LOG(V4L2, Error)
-- 
2.25.1