media/gpu/v4l2: Get V4L2 free buffer with requested buffer ID

The V4L2 stateful API requires the caller pass the same buffers to the
output queue slot. This CL change V4L2Queue::GetFreeBuffer() signature
that allows the caller get a specific output queue slot.

BUG=b:159688625
TEST=video.DecodeAccel.h264 passes on Kukui.

Change-Id: I5245998b236dbb698df3d1ca11b6f269dd5de374
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413943
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Fritz Koenig <frkoenig@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812556}

media/gpu/v4l2/v4l2_device.cc

@@ -285,6 +285,8 @@ class V4L2BuffersList : public base::RefCountedThreadSafe<V4L2BuffersList> {
   void ReturnBuffer(size_t buffer_id);
   // Get any of the buffers in the list. There is no order guarantee whatsoever.
   base::Optional<size_t> GetFreeBuffer();
+  // Get the buffer with specified index.
+  base::Optional<size_t> GetFreeBuffer(size_t requested_buffer_id);
   // Number of buffers currently in this list.
   size_t size() const;
 
@@ -319,6 +321,15 @@ base::Optional<size_t> V4L2BuffersList::GetFreeBuffer() {
   return buffer_id;
 }
 
+base::Optional<size_t> V4L2BuffersList::GetFreeBuffer(
+    size_t requested_buffer_id) {
+  base::AutoLock auto_lock(lock_);
+
+  return (free_buffers_.erase(requested_buffer_id) > 0)
+             ? base::make_optional(requested_buffer_id)
+             : base::nullopt;
+}
+
 size_t V4L2BuffersList::size() const {
   base::AutoLock auto_lock(lock_);
 
@@ -1156,6 +1167,23 @@ base::Optional<V4L2WritableBufferRef> V4L2Queue::GetFreeBuffer() {
       weak_this_factory_.GetWeakPtr());
 }
 
+base::Optional<V4L2WritableBufferRef> V4L2Queue::GetFreeBuffer(
+    size_t requested_buffer_id) {
+  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+
+  // No buffers allocated at the moment?
+  if (!free_buffers_)
+    return base::nullopt;
+
+  auto buffer_id = free_buffers_->GetFreeBuffer(requested_buffer_id);
+  if (!buffer_id.has_value())
+    return base::nullopt;
+
+  return V4L2BufferRefFactory::CreateWritableRef(
+      buffers_[buffer_id.value()]->v4l2_buffer(),
+      weak_this_factory_.GetWeakPtr());
+}
+
 bool V4L2Queue::QueueBuffer(struct v4l2_buffer* v4l2_buffer,
                             scoped_refptr<VideoFrame> video_frame) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

media/gpu/v4l2/v4l2_device.h

@@ -367,6 +367,13 @@ class MEDIA_GPU_EXPORT V4L2Queue
   // If the caller discards the returned reference, the underlying buffer is
   // made available to clients again.
   base::Optional<V4L2WritableBufferRef> GetFreeBuffer();
+  // Return the buffer at index |requested_buffer_id|, if it is available at
+  // this time.
+  //
+  // If the buffer is currently in use or the provided index is invalid,
+  // return |base::nullopt|.
+  base::Optional<V4L2WritableBufferRef> GetFreeBuffer(
+      size_t requested_buffer_id);
 
   // Attempt to dequeue a buffer, and return a reference to it if one was
   // available.