avcodec/v4l2_context: Log warning when all capture buffers are in userspace

v4l2_m2m uses device memory mapped buffers to store dequeued
frames/packets (reference counted by AVBufferRef). When the reference
count drops to zero, the buffer ownership is returned back to the
device, so that they can re-filled with frames/packets.

There are some cases when all the capture buffers are in userspace
(i.e. due to internal buffering in ffmpeg). On the s5p-mfc this causes
an infinite wait when polling to dequeue the buffers, which can be
prevented by increasing the total number of buffers. This commit adds a
warning when all the capture buffers are dequeued.

Reviewed-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/v4l2_context.c

@@ -291,6 +291,18 @@ static V4L2Buffer* v4l2_dequeue_v4l2buf(V4L2Context *ctx, int timeout)
     };
     int i, ret;
 
+    if (!V4L2_TYPE_IS_OUTPUT(ctx->type) && ctx->buffers) {
+        for (i = 0; i < ctx->num_buffers; i++) {
+            if (ctx->buffers[i].status == V4L2BUF_IN_DRIVER)
+                break;
+        }
+        if (i == ctx->num_buffers)
+            av_log(logger(ctx), AV_LOG_WARNING, "All capture buffers returned to "
+                                                "userspace. Increase num_capture_buffers "
+                                                "to prevent device deadlock or dropped "
+                                                "packets/frames.\n");
+    }
+
     /* if we are draining and there are no more capture buffers queued in the driver we are done */
     if (!V4L2_TYPE_IS_OUTPUT(ctx->type) && ctx_to_m2mctx(ctx)->draining) {
         for (i = 0; i < ctx->num_buffers; i++) {