media/gpu/v4l2: add GetCtrl() method to V4L2Device

Add a simple method that allows to query a single control of a
V4L2Device and use it in the V4L2VDA. This will also come handy for the
V4L2VD.

BUG=b:149663704
TEST=VDAtest --use_vd passes on Kukui.

Change-Id: I9e306fcf4115936dde07e8595631b86ceeb6e917
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143425
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#759134}

media/gpu/v4l2/v4l2_device.cc

@@ -2047,6 +2047,23 @@ bool V4L2Device::SetExtCtrls(uint32_t ctrl_class,
   return Ioctl(VIDIOC_S_EXT_CTRLS, &ext_ctrls) == 0;
 }
 
+base::Optional<struct v4l2_ext_control> V4L2Device::GetCtrl(uint32_t ctrl_id) {
+  DCHECK_CALLED_ON_VALID_SEQUENCE(client_sequence_checker_);
+  struct v4l2_ext_control ctrl = {};
+  struct v4l2_ext_controls ext_ctrls = {};
+
+  ctrl.id = ctrl_id;
+  ext_ctrls.controls = &ctrl;
+  ext_ctrls.count = 1;
+
+  if (Ioctl(VIDIOC_G_EXT_CTRLS, &ext_ctrls) != 0) {
+    VPLOGF(3) << "Failed to get control";
+    return base::nullopt;
+  }
+
+  return ctrl;
+}
+
 class V4L2Request {
  public:
   // Apply the passed controls to the request.

media/gpu/v4l2/v4l2_device.h

@@ -726,6 +726,10 @@ class MEDIA_GPU_EXPORT V4L2Device
   // whether the operation succeeded.
   bool SetExtCtrls(uint32_t ctrl_class, std::vector<V4L2ExtCtrl> ctrls);
 
+  // Get the value of a single control, or base::nullopt of the control is not
+  // exposed by the device.
+  base::Optional<struct v4l2_ext_control> GetCtrl(uint32_t ctrl_id);
+
  protected:
   friend class base::RefCountedThreadSafe<V4L2Device>;
   V4L2Device();

media/gpu/v4l2/v4l2_video_decode_accelerator.cc

@@ -2376,11 +2376,10 @@ bool V4L2VideoDecodeAccelerator::CreateOutputBuffers() {
   DCHECK(output_buffer_map_.empty());
 
   // Number of output buffers we need.
-  struct v4l2_control ctrl;
-  memset(&ctrl, 0, sizeof(ctrl));
-  ctrl.id = V4L2_CID_MIN_BUFFERS_FOR_CAPTURE;
-  IOCTL_OR_ERROR_RETURN_FALSE(VIDIOC_G_CTRL, &ctrl);
-  output_dpb_size_ = ctrl.value;
+  auto ctrl = device_->GetCtrl(V4L2_CID_MIN_BUFFERS_FOR_CAPTURE);
+  if (!ctrl)
+    return false;
+  output_dpb_size_ = ctrl->value;
 
   // Output format setup in Initialize().