Markus, Ian,

here a future improvment of this diff, this time I stricly requires that
mmap buffer is switched or "grab" only for the first sample of the frame.

I can't reproduce glitches on my motion test case. May I ask you to try this
one?

Index: sys/dev/usb/uvideo.c
===================================================================
RCS file: /home/cvs/src/sys/dev/usb/uvideo.c,v
diff -u -p -r1.257 uvideo.c
--- sys/dev/usb/uvideo.c	6 Apr 2025 09:46:30 -0000	1.257
+++ sys/dev/usb/uvideo.c	17 Apr 2025 08:00:03 -0000
@@ -66,6 +66,7 @@ struct uvideo_softc {
 	struct uvideo_frame_buffer		 sc_frame_buffer;
 
 	struct uvideo_mmap			 sc_mmap[UVIDEO_MAX_BUFFERS];
+	struct uvideo_mmap			*sc_mmap_cur;
 	uint8_t					*sc_mmap_buffer;
 	size_t					 sc_mmap_buffer_size;
 	int					 sc_mmap_buffer_idx;
@@ -170,8 +171,8 @@ void		uvideo_vs_decode_stream_header(str
 		    uint8_t *, int);
 void		uvideo_vs_decode_stream_header_isight(struct uvideo_softc *,
 		    uint8_t *, int);
-int		uvideo_get_buf(struct uvideo_softc *);
-void		uvideo_mmap_queue(struct uvideo_softc *, uint8_t *, int, int);
+uint8_t *	uvideo_mmap_getbuf(struct uvideo_softc *);
+void		uvideo_mmap_queue(struct uvideo_softc *, int, int);
 void		uvideo_read(struct uvideo_softc *, uint8_t *, int);
 usbd_status	uvideo_usb_control(struct uvideo_softc *, uint8_t, uint8_t,
 		    uint16_t, uint8_t *, size_t);
@@ -2333,6 +2334,7 @@ uvideo_vs_decode_stream_header(struct uv
 	struct uvideo_frame_buffer *fb = &sc->sc_frame_buffer;
 	struct usb_video_stream_header *sh;
 	int sample_len;
+	uint8_t *buf;
 
 	if (frame_size < UVIDEO_SH_MIN_LEN)
 		/* frame too small to contain a valid stream header */
@@ -2364,16 +2366,17 @@ uvideo_vs_decode_stream_header(struct uv
 		fb->fid = sh->bFlags & UVIDEO_SH_FLAG_FID;
 		fb->offset = 0;
 		fb->error = 0;
+	} else if (fb->fid != (sh->bFlags & UVIDEO_SH_FLAG_FID)) {
+		/* first sample for a frame, due to flip FID */
+		DPRINTF(1, "%s: %s: wrong FID, ignore last frame!\n",
+		    DEVNAME(sc), __func__);
+		fb->sample = 1;
+		fb->fid = sh->bFlags & UVIDEO_SH_FLAG_FID;
+		fb->offset = 0;
+		fb->error = 0;
 	} else {
-		/* continues sample for a frame, check consistency */
-		if (fb->fid != (sh->bFlags & UVIDEO_SH_FLAG_FID)) {
-			DPRINTF(1, "%s: %s: wrong FID, ignore last frame!\n",
-			    DEVNAME(sc), __func__);
-			fb->sample = 1;
-			fb->fid = sh->bFlags & UVIDEO_SH_FLAG_FID;
-			fb->offset = 0;
-			fb->error = 0;
-		}
+		/* continues sample for a frame */
+		fb->sample++;
 	}
 
 	if (sh->bFlags & UVIDEO_SH_FLAG_ERR) {
@@ -2382,6 +2385,11 @@ uvideo_vs_decode_stream_header(struct uv
 		fb->error = 1;
 	}
 
+	if (sc->sc_mmap_flag)
+		buf = uvideo_mmap_getbuf(sc);
+	else
+		buf = sc->sc_frame_buffer.buf;
+
 	/* save sample */
 	sample_len = frame_size - sh->bLength;
 	if (sample_len > fb->buf_size - fb->offset) {
@@ -2390,8 +2398,8 @@ uvideo_vs_decode_stream_header(struct uv
 		sample_len = fb->buf_size - fb->offset;
 		fb->error = 1;
 	}
-	if (sample_len > 0) {
-		bcopy(frame + sh->bLength, fb->buf + fb->offset, sample_len);
+	if (buf && sample_len > 0) {
+		bcopy(frame + sh->bLength, buf + fb->offset, sample_len);
 		fb->offset += sample_len;
 	}
 
@@ -2409,7 +2417,7 @@ uvideo_vs_decode_stream_header(struct uv
 
 		if (sc->sc_mmap_flag) {
 			/* mmap */
-			uvideo_mmap_queue(sc, fb->buf, fb->offset, fb->error);
+			uvideo_mmap_queue(sc, fb->offset, fb->error);
 		} else if (fb->error) {
 			DPRINTF(1, "%s: %s: error frame, skipped!\n",
 			    DEVNAME(sc), __func__);
@@ -2446,6 +2454,7 @@ uvideo_vs_decode_stream_header_isight(st
 {
 	struct uvideo_frame_buffer *fb = &sc->sc_frame_buffer;
 	int sample_len, header = 0;
+	uint8_t *buf;
 	uint8_t magic[] = {
 	    0x11, 0x22, 0x33, 0x44,
 	    0xde, 0xad, 0xbe, 0xef, 0xde, 0xad, 0xfa, 0xce };
@@ -2463,27 +2472,52 @@ uvideo_vs_decode_stream_header_isight(st
 	if (header) {
 		if (sc->sc_mmap_flag) {
 			/* mmap */
-			uvideo_mmap_queue(sc, fb->buf, fb->offset, 0);
+			uvideo_mmap_queue(sc, fb->offset, 0);
 		} else {
 			/* read */
 			uvideo_read(sc, fb->buf, fb->offset);
 		}
 		fb->offset = 0;
 	} else {
+		if (sc->sc_mmap_flag)
+			buf = uvideo_mmap_getbuf(sc);
+		else
+			buf = sc->sc_frame_buffer.buf;
+
+		if (buf == NULL)
+			return;
+
 		/* save sample */
 		sample_len = frame_size;
 		if ((fb->offset + sample_len) <= fb->buf_size) {
-			bcopy(frame, fb->buf + fb->offset, sample_len);
+			bcopy(frame, buf + fb->offset, sample_len);
 			fb->offset += sample_len;
 		}
 	}
 }
 
-int
-uvideo_get_buf(struct uvideo_softc *sc)
+uint8_t *
+uvideo_mmap_getbuf(struct uvideo_softc *sc)
 {
 	int i, idx = sc->sc_mmap_buffer_idx;
 
+	/*
+	 * Section 2.4.3.2 explicitly allows multiple frames per one
+	 * transfer and multiple transfers per one frame.
+	 */
+	if (sc->sc_mmap_cur != NULL)
+		return sc->sc_mmap_cur->buf;
+
+	/* use the next buffer only for the first frame's sample */
+	if (sc->sc_frame_buffer.sample > 1) {
+		DPRINTF(1, "%s: %s: not the first sample: %d\n",
+		    DEVNAME(sc), __func__, sc->sc_frame_buffer.sample);
+		return NULL;
+	}
+
+	if (sc->sc_mmap_count == 0 || sc->sc_mmap_buffer == NULL)
+		panic("%s: mmap buffers not allocated", __func__);
+
 	/* find a buffer which is ready for queueing */
 	for (i = 0; i < sc->sc_mmap_count; i++) {
 		if (sc->sc_mmap[sc->sc_mmap_buffer_idx].v4l2_buf.flags &
@@ -2497,51 +2531,45 @@ uvideo_get_buf(struct uvideo_softc *sc)
 			sc->sc_mmap_buffer_idx = 0;
 	}
 
-	if (i == sc->sc_mmap_count)
-		return -1;
+	if (i == sc->sc_mmap_count) {
+		DPRINTF(1, "%s: %s: mmap queue is full!\n",
+		    DEVNAME(sc), __func__);
+		return NULL;
+	}
+
+	sc->sc_mmap_cur = &sc->sc_mmap[idx];
 
-	return idx;
+	return sc->sc_mmap_cur->buf;
 }
 
 void
-uvideo_mmap_queue(struct uvideo_softc *sc, uint8_t *buf, int len, int err)
+uvideo_mmap_queue(struct uvideo_softc *sc, int len, int err)
 {
-	int i;
-
-	if (sc->sc_mmap_count == 0 || sc->sc_mmap_buffer == NULL)
-		panic("%s: mmap buffers not allocated", __func__);
-
-	/* find a buffer which is ready for queueing */
-	i = uvideo_get_buf(sc);
-	if (i == -1) {
-		DPRINTF(1, "%s: %s: mmap queue is full!\n",
-		    DEVNAME(sc), __func__);
+	if (sc->sc_mmap_cur == NULL)
 		return;
-	}
 
-	/* copy frame to mmap buffer and report length */
-	bcopy(buf, sc->sc_mmap[i].buf, len);
-	sc->sc_mmap[i].v4l2_buf.bytesused = len;
+	/* report frame length */
+	sc->sc_mmap_cur->v4l2_buf.bytesused = len;
 
 	/* timestamp it */
-	getmicrouptime(&sc->sc_mmap[i].v4l2_buf.timestamp);
-	sc->sc_mmap[i].v4l2_buf.flags &= ~V4L2_BUF_FLAG_TIMESTAMP_MASK;
-	sc->sc_mmap[i].v4l2_buf.flags |= V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
-	sc->sc_mmap[i].v4l2_buf.flags &= ~V4L2_BUF_FLAG_TSTAMP_SRC_MASK;
-	sc->sc_mmap[i].v4l2_buf.flags |= V4L2_BUF_FLAG_TSTAMP_SRC_EOF;
-	sc->sc_mmap[i].v4l2_buf.flags &= ~V4L2_BUF_FLAG_TIMECODE;
+	getmicrouptime(&sc->sc_mmap_cur->v4l2_buf.timestamp);
+	sc->sc_mmap_cur->v4l2_buf.flags &= ~V4L2_BUF_FLAG_TIMESTAMP_MASK;
+	sc->sc_mmap_cur->v4l2_buf.flags |= V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
+	sc->sc_mmap_cur->v4l2_buf.flags &= ~V4L2_BUF_FLAG_TSTAMP_SRC_MASK;
+	sc->sc_mmap_cur->v4l2_buf.flags |= V4L2_BUF_FLAG_TSTAMP_SRC_EOF;
+	sc->sc_mmap_cur->v4l2_buf.flags &= ~V4L2_BUF_FLAG_TIMECODE;
 
 	/* forward error bit */
-	sc->sc_mmap[i].v4l2_buf.flags &= ~V4L2_BUF_FLAG_ERROR;
+	sc->sc_mmap_cur->v4l2_buf.flags &= ~V4L2_BUF_FLAG_ERROR;
 	if (err)
-		sc->sc_mmap[i].v4l2_buf.flags |= V4L2_BUF_FLAG_ERROR;
+		sc->sc_mmap_cur->v4l2_buf.flags |= V4L2_BUF_FLAG_ERROR;
 
 	/* queue it */
-	sc->sc_mmap[i].v4l2_buf.flags |= V4L2_BUF_FLAG_DONE;
-	sc->sc_mmap[i].v4l2_buf.flags &= ~V4L2_BUF_FLAG_QUEUED;
-	SIMPLEQ_INSERT_TAIL(&sc->sc_mmap_q, &sc->sc_mmap[i], q_frames);
-	DPRINTF(2, "%s: %s: frame queued on index %d\n",
-	    DEVNAME(sc), __func__, i);
+	sc->sc_mmap_cur->v4l2_buf.flags |= V4L2_BUF_FLAG_DONE;
+	sc->sc_mmap_cur->v4l2_buf.flags &= ~V4L2_BUF_FLAG_QUEUED;
+	SIMPLEQ_INSERT_TAIL(&sc->sc_mmap_q, sc->sc_mmap_cur, q_frames);
+	sc->sc_mmap_cur = NULL;
+	DPRINTF(2, "%s: %s: frame queued\n", DEVNAME(sc), __func__);
 
 	wakeup(sc);
 
@@ -3507,6 +3535,7 @@ uvideo_reqbufs(void *v, struct v4l2_requ
 	}
 
 	sc->sc_mmap_buffer_idx = 0;
+	sc->sc_mmap_cur = NULL;
 
 	/* tell how many buffers we have really allocated */
 	rb->count = sc->sc_mmap_count;