v4l2: Allocate extra buffers for the rest of the pipeline

When allocating buffers for the output (capture) queue with the new VD,
we should add extra buffers to what the driver says we need to account
for delays with the rest of Chrome's rendering pipeline.

BUG=b:165647432,b:157703889
TEST=Playback no longer stutters on trogdor

Change-Id: I782377dd10e3ac0c63ce12d12b6ac3c49c370b8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2433591
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Auto-Submit: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811508}

media/gpu/v4l2/v4l2_video_decoder.cc

@@ -11,6 +11,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/task/post_task.h"
+#include "media/base/limits.h"
 #include "media/base/video_types.h"
 #include "media/base/video_util.h"
 #include "media/gpu/chromeos/dmabuf_video_frame_pool.h"
@@ -38,6 +39,11 @@ constexpr uint32_t kSupportedInputFourccs[] = {
     V4L2_PIX_FMT_H264,       V4L2_PIX_FMT_VP8,       V4L2_PIX_FMT_VP9,
 };
 
+// Number of output buffers to use for each VD stage above what's required by
+// the decoder (e.g. DPB size, in H264).  We need limits::kMaxVideoFrames to
+// fill up the GpuVideoDecode pipeline, and +1 for a frame in transit.
+constexpr size_t kDpbOutputBufferExtraCount = limits::kMaxVideoFrames + 1;
+
 }  // namespace
 
 // static
@@ -514,7 +520,8 @@ void V4L2VideoDecoder::ContinueChangeResolution(
       base::BindOnce(&V4L2VideoDecoderBackend::OnChangeResolutionDone,
                      base::Unretained(backend_.get()), false));
 
-  num_output_frames_ = num_output_frames;
+  DCHECK_GT(num_output_frames, 0u);
+  num_output_frames_ = num_output_frames + kDpbOutputBufferExtraCount;
 
   // Stateful decoders require the input queue to keep running during resolution
   // changes, but stateless ones require it to be stopped.
@@ -525,9 +532,8 @@ void V4L2VideoDecoder::ContinueChangeResolution(
     SetState(State::kError);
     return;
   }
-  DCHECK_GT(num_output_frames, 0u);
 
-  if (!backend_->ApplyResolution(pic_size, visible_rect, num_output_frames)) {
+  if (!backend_->ApplyResolution(pic_size, visible_rect, num_output_frames_)) {
     SetState(State::kError);
     return;
   }

media/gpu/v4l2/v4l2_video_decoder_backend_stateful.cc

@@ -498,7 +498,7 @@ void V4L2StatefulVideoDecoderBackend::ChangeResolution() {
   }
 
   auto ctrl = device_->GetCtrl(V4L2_CID_MIN_BUFFERS_FOR_CAPTURE);
-  constexpr size_t DEFAULT_NUM_OUTPUT_BUFFERS = 12;
+  constexpr size_t DEFAULT_NUM_OUTPUT_BUFFERS = 7;
   const size_t num_output_buffers =
       ctrl ? ctrl->value : DEFAULT_NUM_OUTPUT_BUFFERS;
   if (!ctrl)