v4l2: allow to convert monotonic timestamps.

configure

@@ -1157,6 +1157,7 @@ HAVE_LIST="
     attribute_may_alias
     attribute_packed
     cbrtf
+    clock_gettime
     closesocket
     cmov
     dcbzl
@@ -3048,6 +3049,7 @@ fi
 # Solaris has nanosleep in -lrt, OpenSolaris no longer needs that
 check_func nanosleep || { check_func nanosleep -lrt && add_extralibs -lrt; }
 
+check_func  clock_gettime || { check_func clock_gettime -lrt && add_extralibs -lrt; }
 check_func  fcntl
 check_func  fork
 check_func  getaddrinfo $network_extralibs

doc/indevs.texi

@@ -520,6 +520,13 @@ supported using @command{-list_formats all} for Video4Linux2 devices.
 
 Some usage examples of the video4linux2 devices with ffmpeg and ffplay:
 
+The time base for the timestamps is 1 microsecond. Depending on the kernel
+version and configuration, the timestamps may be derived from the real time
+clock (origin at the Unix Epoch) or the monotonic clock (origin usually at
+boot time, unaffected by NTP or manual changes to the clock). The
+@option{-timestamps abs} or @option{-ts abs} option can be used to force
+conversion into the real time clock.
+
 Note that if FFmpeg is build with v4l-utils support ("--enable-libv4l2"
 option), it will always be used.
 @example

libavdevice/Makefile

@@ -28,7 +28,7 @@ OBJS-$(CONFIG_PULSE_INDEV)               += pulse.o
 OBJS-$(CONFIG_SDL_OUTDEV)                += sdl.o
 OBJS-$(CONFIG_SNDIO_INDEV)               += sndio_common.o sndio_dec.o
 OBJS-$(CONFIG_SNDIO_OUTDEV)              += sndio_common.o sndio_enc.o
-OBJS-$(CONFIG_V4L2_INDEV)                += v4l2.o
+OBJS-$(CONFIG_V4L2_INDEV)                += v4l2.o timefilter.o
 OBJS-$(CONFIG_V4L_INDEV)                 += v4l.o
 OBJS-$(CONFIG_VFWCAP_INDEV)              += vfwcap.o
 OBJS-$(CONFIG_X11_GRAB_DEVICE_INDEV)     += x11grab.o

libavdevice/v4l2.c

@@ -51,6 +51,7 @@
 #include "libavutil/log.h"
 #include "libavutil/opt.h"
 #include "avdevice.h"
+#include "timefilter.h"
 #include "libavutil/parseutils.h"
 #include "libavutil/pixdesc.h"
 #include "libavutil/avstring.h"
@@ -73,6 +74,28 @@ static const int desired_video_buffers = 256;
 #define V4L_RAWFORMATS  1
 #define V4L_COMPFORMATS 2
 
+/**
+ * Return timestamps to the user exactly as returned by the kernel
+ */
+#define V4L_TS_DEFAULT  0
+/**
+ * Autodetect the kind of timestamps returned by the kernel and convert to
+ * absolute (wall clock) timestamps.
+ */
+#define V4L_TS_ABS      1
+/**
+ * Assume kernel timestamps are from the monotonic clock and convert to
+ * absolute timestamps.
+ */
+#define V4L_TS_MONO2ABS 2
+
+/**
+ * Once the kind of timestamps returned by the kernel have been detected,
+ * the value of the timefilter (NULL or not) determines whether a conversion
+ * takes place.
+ */
+#define V4L_TS_CONVERT_READY V4L_TS_DEFAULT
+
 struct video_data {
     AVClass *class;
     int fd;
@@ -81,6 +104,9 @@ struct video_data {
     int frame_size;
     int interlaced;
     int top_field_first;
+    int ts_mode;
+    TimeFilter *timefilter;
+    int64_t last_time_m;
 
     int buffers;
     void **buf_start;
@@ -455,6 +481,66 @@ static void mmap_release_buffer(AVPacket *pkt)
     pkt->size = 0;
 }
 
+#if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
+static int64_t av_gettime_monotonic(void)
+{
+    struct timespec tv;
+
+    clock_gettime(CLOCK_MONOTONIC, &tv);
+    return (int64_t)tv.tv_sec * 1000000 + tv.tv_nsec / 1000;
+}
+#endif
+
+static int init_convert_timestamp(AVFormatContext *ctx, int64_t ts)
+{
+    struct video_data *s = ctx->priv_data;
+    int64_t now;
+
+    now = av_gettime();
+    if (s->ts_mode == V4L_TS_ABS &&
+        ts <= now + 1 * AV_TIME_BASE && ts >= now - 10 * AV_TIME_BASE) {
+        av_log(ctx, AV_LOG_INFO, "Detected absolute timestamps\n");
+        s->ts_mode = V4L_TS_CONVERT_READY;
+        return 0;
+    }
+#if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
+    now = av_gettime_monotonic();
+    if (s->ts_mode == V4L_TS_MONO2ABS ||
+        (ts <= now + 1 * AV_TIME_BASE && ts >= now - 10 * AV_TIME_BASE)) {
+        int64_t period = av_rescale_q(1, ctx->streams[0]->codec->time_base,
+                                      AV_TIME_BASE_Q);
+        av_log(ctx, AV_LOG_INFO, "Detected monotonic timestamps, converting\n");
+        /* microseconds instead of seconds, MHz instead of Hz */
+        s->timefilter = ff_timefilter_new(1, period, 1.0E-6);
+        s->ts_mode = V4L_TS_CONVERT_READY;
+        return 0;
+    }
+#endif
+    av_log(ctx, AV_LOG_ERROR, "Unknown timestamps\n");
+    return AVERROR(EIO);
+}
+
+static int convert_timestamp(AVFormatContext *ctx, int64_t *ts)
+{
+    struct video_data *s = ctx->priv_data;
+
+    if (s->ts_mode) {
+        int r = init_convert_timestamp(ctx, *ts);
+        if (r < 0)
+            return r;
+    }
+#if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
+    if (s->timefilter) {
+        int64_t nowa = av_gettime();
+        int64_t nowm = av_gettime_monotonic();
+        ff_timefilter_update(s->timefilter, nowa, nowm - s->last_time_m);
+        s->last_time_m = nowm;
+        *ts = ff_timefilter_eval(s->timefilter, *ts - nowm);
+    }
+#endif
+    return 0;
+}
+
 static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
 {
     struct video_data *s = ctx->priv_data;
@@ -490,6 +576,9 @@ static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
     pkt->data= s->buf_start[buf.index];
     pkt->size = buf.bytesused;
     pkt->pts = buf.timestamp.tv_sec * INT64_C(1000000) + buf.timestamp.tv_usec;
+    res = convert_timestamp(ctx, &pkt->pts);
+    if (res < 0)
+        return res;
     pkt->destruct = mmap_release_buffer;
     buf_descriptor = av_malloc(sizeof(struct buff_data));
     if (buf_descriptor == NULL) {
@@ -847,6 +936,11 @@ static const AVOption options[] = {
     { "all",          "Show all available formats",                                OFFSET(list_format),  AV_OPT_TYPE_CONST,  {.dbl = V4L_ALLFORMATS  },    0, INT_MAX, DEC, "list_formats" },
     { "raw",          "Show only non-compressed formats",                          OFFSET(list_format),  AV_OPT_TYPE_CONST,  {.dbl = V4L_RAWFORMATS  },    0, INT_MAX, DEC, "list_formats" },
     { "compressed",   "Show only compressed formats",                              OFFSET(list_format),  AV_OPT_TYPE_CONST,  {.dbl = V4L_COMPFORMATS },    0, INT_MAX, DEC, "list_formats" },
+    { "timestamps",   "Kind of timestamps for grabbed frames",                     OFFSET(ts_mode),      AV_OPT_TYPE_INT,    {.dbl = 0 }, 0, 2, DEC, "timestamps" },
+    { "default",      "Use timestamps from the kernel",                            OFFSET(ts_mode),      AV_OPT_TYPE_CONST,  {.dbl = V4L_TS_DEFAULT  }, 0, 2, DEC, "timestamps" },
+    { "abs",          "Use absolute timestamps (wall clock)",                      OFFSET(ts_mode),      AV_OPT_TYPE_CONST,  {.dbl = V4L_TS_ABS      }, 0, 2, DEC, "timestamps" },
+    { "mono2abs",     "Force conversion from monotonic to absolute timestamps",    OFFSET(ts_mode),      AV_OPT_TYPE_CONST,  {.dbl = V4L_TS_MONO2ABS }, 0, 2, DEC, "timestamps" },
+    { "ts",           "Kind of timestamps for grabbed frames",                     OFFSET(ts_mode),      AV_OPT_TYPE_INT,    {.dbl = 0 }, 0, 2, DEC, "timestamps" },
     { NULL },
 };