media/gpu/test: Add documentation about the new video decoder perf tests.

This CL adds documentation for the new video decoder performance tests
(video_decode_accelerator_perf_tests).

TEST=None

BUG=925714

Change-Id: I04a6d5cd348cb05da92b0a4cc44721e3c58ea807
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1563530
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651076}

docs/README.md

@@ -291,6 +291,8 @@ used when committed.
     use the accelerated video decoder/encoder test programs.
 *   [Video Decoder Tests](media/gpu/video_decoder_test_usage.md) - Running the
     new video decoder tests.
+*   [Video Decoder Performance Tests](media/gpu/video_decoder_perf_test_usage.md) -
+    Running the new video decoder performance tests.
 
 ### Accessibility
 *   [Accessibility Overview](accessibility/overview.md) - Overview of

docs/media/gpu/video_decoder_perf_test_usage.md

@@ -0,0 +1,66 @@
+# Video Decoder performance tests
+The video decoder performance tests are a set of tests used to measure the
+performance of various video decoder implementations. These tests run directly
+on top of the video decoder implementation, and don't require the full Chrome
+browser stack. They are build on top of the
+[GoogleTest](https://github.com/google/googletest/blob/master/README.md)
+framework.
+
+[TOC]
+
+## Running from Tast
+__Note:__ The video decoder performance tests are in the progress of being added
+to the Tast framework. This section will be updated once this is done.
+
+## Running manually
+To run the video decoder performance tests manually the
+_video_decode_accelerator_perf_tests_ target needs to be built and deployed to
+the device being tested. Running the video decoder performance tests can be done
+by executing:
+
+    ./video_decode_accelerator_perf_tests [<video path>] [<video metadata path>]
+
+e.g.: `./video_decode_accelerator_perf_tests test-25fps.h264`
+
+__Test videos:__ Test videos are present for multiple codecs in the
+[_media/test/data_](https://cs.chromium.org/chromium/src/media/test/data/)
+folder in Chromium's source tree (e.g.
+[_test-25fps.vp8_](https://cs.chromium.org/chromium/src/media/test/data/test-25fps.vp8)).
+If no video is specified _test-25fps.h264_ will be used.
+
+__Video Metadata:__ These videos also have an accompanying metadata _.json_ file
+that needs to be deployed alongside the test video. They can also be found in
+the _media/test/data_ folder (e.g.
+[_test-25fps.h264.json_](https://cs.chromium.org/chromium/src/media/test/data/test-25fps.h264.json)).
+If no metadata file is specified _\<video path\>.json_ will be used. The video
+metadata file contains info about the video such as its codec profile,
+dimensions and number of frames.
+
+__Note:__ Various CPU scaling and thermal throttling mechanisms might be active
+on the device (e.g. _scaling_governor_, _intel_pstate_). As these can influence
+test results it's advised to disable them.
+
+## Performance metrics
+Currently uncapped decoder performance is measured by playing the specified test
+video from start to finish. Various performance metrics (e.g. the number of
+frames decoded per second) are collected, and written to a file called
+_perf_metrics/<test_name>.txt_. Individual frame decode times can be found in
+_perf_metrics/<test_name>.frame_times.txt_.
+
+__Note:__ A capped performance test is in the process of being added. This test
+will simulate a more realistic real-time decoding environment.
+
+## Command line options
+Multiple command line arguments can be given to the command:
+
+     -v                  enable verbose mode, e.g. -v=2.
+    --vmodule            enable verbose mode for the specified module,
+                         e.g. --vmodule=*media/gpu*=2.
+    --use_vd             use the new VD-based video decoders, instead of
+                         the default VDA-based video decoders.
+    --gtest_help         display the gtest help and exit.
+    --help               display this help and exit.
+
+## Source code
+See the video decoder performance tests [source code](https://cs.chromium.org/chromium/src/media/gpu/video_decode_accelerator_perf_tests.cc).
+

media/gpu/video_decode_accelerator_perf_tests.cc

@@ -20,7 +20,9 @@ namespace test {
 
 namespace {
 
-// Video decoder perf tests usage message.
+// Video decoder perf tests usage message. Make sure to also update the
+// documentation under docs/media/gpu/video_decoder_perf_test_usage.md when
+// making changes here.
 constexpr const char* usage_msg =
     "usage: video_decode_accelerator_perf_tests\n"
     "           [-v=<level>] [--vmodule=<config>] [--use_vd] [--gtest_help]\n"