media/gpu/v4l2: add and use V4L2Device::DequeueEvent()

Add a method to try and dequeue and event from a V4L2 device, so clients
don't need to call the ioctl directly.

BUG=b:149663704
TEST=vdatest passing on Hana.

Change-Id: Ib6a30f3003dde603b5dee2e9c2a7c26ea02ba260
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2131432Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756627}

media/gpu/v4l2/v4l2_device.cc

@@ -1990,6 +1990,18 @@ void V4L2Device::SchedulePoll() {
   device_poller_->SchedulePoll();
 }
 
+base::Optional<struct v4l2_event> V4L2Device::DequeueEvent() {
+  DCHECK_CALLED_ON_VALID_SEQUENCE(client_sequence_checker_);
+  struct v4l2_event event = {};
+
+  if (Ioctl(VIDIOC_DQEVENT, &event) != 0) {
+    VPLOGF(3) << "Failed to dequeue event";
+    return base::nullopt;
+  }
+
+  return event;
+}
+
 V4L2RequestsQueue* V4L2Device::GetRequestsQueue() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(client_sequence_checker_);
 

media/gpu/v4l2/v4l2_device.h

@@ -713,6 +713,9 @@ class MEDIA_GPU_EXPORT V4L2Device
   // to be called from V4L2Queue, clients should not need to call it directly.
   void SchedulePoll();
 
+  // Attempt to dequeue a V4L2 event and return it.
+  base::Optional<struct v4l2_event> DequeueEvent();
+
   // Returns requests queue to get free requests. A null pointer is returned if
   // the queue creation failed or if requests are not supported.
   V4L2RequestsQueue* GetRequestsQueue();

media/gpu/v4l2/v4l2_mjpeg_decode_accelerator.cc

@@ -756,18 +756,16 @@ void V4L2MjpegDecodeAccelerator::DevicePollTask() {
 bool V4L2MjpegDecodeAccelerator::DequeueSourceChangeEvent() {
   DCHECK(decoder_task_runner_->BelongsToCurrentThread());
 
-  struct v4l2_event ev;
-  memset(&ev, 0, sizeof(ev));
-
-  if (device_->Ioctl(VIDIOC_DQEVENT, &ev) == 0) {
-    if (ev.type == V4L2_EVENT_SOURCE_CHANGE) {
-      VLOGF(2) << ": got source change event: " << ev.u.src_change.changes;
-      if (ev.u.src_change.changes & V4L2_EVENT_SRC_CH_RESOLUTION) {
+  if (base::Optional<struct v4l2_event> event = device_->DequeueEvent()) {
+    if (event->type == V4L2_EVENT_SOURCE_CHANGE) {
+      VLOGF(2) << ": got source change event: " << event->u.src_change.changes;
+      if (event->u.src_change.changes & V4L2_EVENT_SRC_CH_RESOLUTION) {
         return true;
       }
       VLOGF(1) << "unexpected source change event.";
     } else {
-      VLOGF(1) << "got an event (" << ev.type << ") we haven't subscribed to.";
+      VLOGF(1) << "got an event (" << event->type
+               << ") we haven't subscribed to.";
     }
   } else {
     VLOGF(1) << "dequeue event failed.";

media/gpu/v4l2/v4l2_video_decode_accelerator.cc

@@ -1391,17 +1391,15 @@ bool V4L2VideoDecodeAccelerator::DequeueResolutionChangeEvent() {
   DCHECK_NE(decoder_state_, kUninitialized);
   DVLOGF(3);
 
-  struct v4l2_event ev;
-  memset(&ev, 0, sizeof(ev));
-
-  while (device_->Ioctl(VIDIOC_DQEVENT, &ev) == 0) {
-    if (ev.type == V4L2_EVENT_SOURCE_CHANGE) {
-      if (ev.u.src_change.changes & V4L2_EVENT_SRC_CH_RESOLUTION) {
+  while (base::Optional<struct v4l2_event> event = device_->DequeueEvent()) {
+    if (event->type == V4L2_EVENT_SOURCE_CHANGE) {
+      if (event->u.src_change.changes & V4L2_EVENT_SRC_CH_RESOLUTION) {
         VLOGF(2) << "got resolution change event.";
         return true;
       }
     } else {
-      VLOGF(1) << "got an event (" << ev.type << ") we haven't subscribed to.";
+      VLOGF(1) << "got an event (" << event->type
+               << ") we haven't subscribed to.";
     }
   }
   return false;