media: Add documentation about logging

Change-Id: Idda24d4e408e257fe1da332e7869b0b708f80a7f
Reviewed-on: https://chromium-review.googlesource.com/1056209
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558102}

media/README.md

@@ -151,3 +151,24 @@ issues, it's helpful to review the internal logs at chrome://media-internals.
 The internals page contains information about active
 `media::WebMediaPlayerImpl`, `media::AudioInputController`,
 `media::AudioOutputController`, and `media::AudioOutputStream` instances.
+
+
+
+# Logging
+
+Media playback typically involves multiple threads, in many cases even multiple
+processes. Media operations are often asynchronous running in a sandbox. These
+make attaching a debugger (e.g. GDB) sometimes less efficient than other
+mechanisms like logging.
+
+In media we use DVLOG() a lot. It makes filename-based filtering super easy.
+Within one file, not all logs are created equal. To make log filtering
+more convenient, use appropriate log levels. Here are some general
+recommendations:
+
+* DVLOG(1): Once per playback events or other important events, e.g.
+  construction/destruction, initialization, playback start/end, suspend/resume,
+  any error conditions.
+* DVLOG(2): Recurring events per playback, e.g. seek/reset/flush, config change.
+* DVLOG(3): Frequent events, e.g. demuxer read, audio/video buffer decrypt or
+  decode, audio/video frame rendering.