diff --git a/media/codec2/sfplugin/PipelineWatcher.cpp b/media/codec2/sfplugin/PipelineWatcher.cpp
index cdcc41b..df81d49 100644
--- a/media/codec2/sfplugin/PipelineWatcher.cpp
+++ b/media/codec2/sfplugin/PipelineWatcher.cpp
@@ -139,8 +139,8 @@
               std::chrono::duration_cast<std::chrono::milliseconds>(elapsed).count());
         durations.push_back(elapsed);
     }
-    nth_element(durations.begin(), durations.end(), durations.begin() + n,
-                std::greater<Clock::duration>());
+    std::nth_element(durations.begin(), durations.end(), durations.begin() + n,
+                     std::greater<Clock::duration>());
     return durations[n];
 }
 
diff --git a/media/codec2/sfplugin/PipelineWatcher.h b/media/codec2/sfplugin/PipelineWatcher.h
index 1c127e4..1e23147 100644
--- a/media/codec2/sfplugin/PipelineWatcher.h
+++ b/media/codec2/sfplugin/PipelineWatcher.h
@@ -26,7 +26,8 @@
 namespace android {
 
 /**
- * PipelineWatcher watches the status of the work.
+ * PipelineWatcher watches the pipeline and infers the status of work items from
+ * events.
  */
 class PipelineWatcher {
 public:
@@ -39,21 +40,81 @@
           mSmoothnessFactor(0) {}
     ~PipelineWatcher() = default;
 
+    /**
+     * \param value the new input delay value
+     * \return  this object
+     */
     PipelineWatcher &inputDelay(uint32_t value);
+
+    /**
+     * \param value the new pipeline delay value
+     * \return  this object
+     */
     PipelineWatcher &pipelineDelay(uint32_t value);
+
+    /**
+     * \param value the new output delay value
+     * \return  this object
+     */
     PipelineWatcher &outputDelay(uint32_t value);
+
+    /**
+     * \param value the new smoothness factor value
+     * \return  this object
+     */
     PipelineWatcher &smoothnessFactor(uint32_t value);
 
+    /**
+     * Client queued a work item to the component.
+     *
+     * \param frameIndex  input frame index of this work
+     * \param buffers     input buffers of the queued work item
+     * \param queuedAt    time when the client queued the buffer
+     */
     void onWorkQueued(
             uint64_t frameIndex,
             std::vector<std::shared_ptr<C2Buffer>> &&buffers,
             const Clock::time_point &queuedAt);
+
+    /**
+     * The component released input buffers from a work item.
+     *
+     * \param frameIndex  input frame index
+     * \param arrayIndex  index of the buffer at the original |buffers| in
+     *                    onWorkQueued().
+     * \return  buffers[arrayIndex]
+     */
     std::shared_ptr<C2Buffer> onInputBufferReleased(
             uint64_t frameIndex, size_t arrayIndex);
+
+    /**
+     * The component finished processing a work item.
+     *
+     * \param frameIndex  input frame index
+     */
     void onWorkDone(uint64_t frameIndex);
+
+    /**
+     * Flush the pipeline.
+     */
     void flush();
 
+    /**
+     * \return  true  if pipeline does not need more work items to proceed
+     *                smoothly, considering delays and smoothness factor;
+     *          false otherwise.
+     */
     bool pipelineFull() const;
+
+    /**
+     * Return elapsed processing time of a work item, nth from the longest
+     * processing time to the shortest.
+     *
+     * \param now current timestamp
+     * \param n   nth work item, from the longest processing time to the
+     *            shortest. It's a 0-based index.
+     * \return  elapsed processing time of nth work item.
+     */
     Clock::duration elapsed(const Clock::time_point &now, size_t n) const;
 
 private: