V4L2Decoder: Use TRY_DECODER_CMD when checking flush support

Commit 05617941 ("V4L2Device: Refactor querying capabilities")
changed behaviour when testing if V4L2_DEC_CMD_STOP is supported
by the device.

Before, it used VIDIOC_TRY_DECODER_CMD
Now, it uses VIDIOC_DECODER_CMD

According to the kernel docs [1], VIDIOC_TRY_DECODER_CMD can be used to
try a command without actually executing it.

In V4L2Decoder::start(), it seems we want to try if V4L2_DEC_CMD_STOP is
supported, not actually executing it.

Introduce a new helper function, tryV4L2DecoderCmd() to do so and use
that instead.

[1] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/vidioc-decoder-cmd.html#description
Fixes: 05617941 ("V4L2Device: Refactor querying capabilities")
Link: https://android-review.googlesource.com/c/platform/external/v4l2_codec2/+/3301216


Change-Id: I523baa430951c65aa904cdb9f5636bb424ab2dd6
Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

v4l2/V4L2Decoder.cpp

@@ -144,7 +144,7 @@ bool V4L2Decoder::start(const VideoCodec& codec, const size_t inputBufferSize,
         return false;
     }
 
-    if (!sendV4L2DecoderCmd(false)) {
+    if (!tryV4L2DecoderCmd(false)) {
         ALOGE("Device does not support flushing (V4L2_DEC_CMD_STOP)");
         return false;
     }
@@ -986,6 +986,20 @@ bool V4L2Decoder::sendV4L2DecoderCmd(bool start) {
     return true;
 }
 
+bool V4L2Decoder::tryV4L2DecoderCmd(bool start) {
+    ALOGV("%s(start=%d)", __func__, start);
+    ALOG_ASSERT(mTaskRunner->RunsTasksInCurrentSequence());
+
+    struct v4l2_decoder_cmd cmd;
+    memset(&cmd, 0, sizeof(cmd));
+    cmd.cmd = start ? V4L2_DEC_CMD_START : V4L2_DEC_CMD_STOP;
+    if (mDevice->ioctl(VIDIOC_TRY_DECODER_CMD, &cmd) != 0) {
+        return false;
+    }
+
+    return true;
+}
+
 void V4L2Decoder::onError() {
     ALOGV("%s()", __func__);
     ALOG_ASSERT(mTaskRunner->RunsTasksInCurrentSequence());

v4l2/include/v4l2_codec2/v4l2/V4L2Decoder.h

@@ -95,6 +95,7 @@ private:
     std::optional<struct v4l2_format> getFormatInfo();
     Rect getVisibleRect(const ui::Size& codedSize);
     bool sendV4L2DecoderCmd(bool start);
+    bool tryV4L2DecoderCmd(bool start);
 
     void setState(State newState);
     void onError();