derived: add DerivedSurfaceFlingerStats

Add DerivedSurfaceFlingerStats that parse output from
SurfaceFlingerFramesInstrument to produce FPS data and rendering
statistics.

doc/derived_measurements.rst

@@ -179,3 +179,43 @@ FPS / Rendering
    Please see the `gfxinfo documentation`_ for details.
 
 .. _gfxinfo documentation: https://developer.android.com/training/testing/performance.html
+
+
+.. class:: DerivedSurfaceFlingerStats(drop_threshold=5, suffix='-fps', filename=None, outdir=None)
+
+   Produces FPS (frames-per-second) and other dervied statistics from
+   :class:`SurfaceFlingerFramesInstrument` output. This takes several optional
+   parameters in creation:
+
+   :param drop_threshold: FPS in an application, such as a game, which this
+                          processor is primarily targeted at, cannot reasonably
+                          drop to a very low value. This is specified to this
+                          threhold. If an FPS for a frame is computed to be
+                          lower than this treshold, it will be dropped on the
+                          assumption that frame rednering was suspended by the
+                          system (e.g. when idling), or there was some sort of
+                          error, and therefore this should be used in
+                          performance calculations. defaults to ``5``.
+   :param  suffix: The name of the gerated per-frame FPS csv file will be
+                   derived from the input frames csv file by appending this
+                   suffix. This cannot be specified at the same time as
+                   a ``filename``.
+   :param filename: As an alternative to the suffix, a complete file name for
+                    FPS csv can be specified. This cannot be used at the same
+                    time as the ``suffix``.
+   :param outdir: By default, the FPS csv file will be placed in the same
+                  directory as the input frames csv file. This can be changed
+                  by specifying an alternate directory here
+
+   .. warning:: Specifying both ``filename`` and ``oudir`` will mean that exactly
+                the same file will be used for FPS output on each invocation of
+                ``process()`` (even for different inputs) resulting in previous
+                results being overwritten.
+
+.. method:: DerivedSurfaceFlingerStats.process(measurement_csv)
+
+   Process the fames csv generated by :class:`SurfaceFlingerFramesInstrument` and
+   returns a list containing exactly three entries: :class:`DerivedMetric`\ s
+   ``fps`` and ``total_frames``, followed by a :class:`MeasurentCsv` containing
+   per-frame FPSs values, followed by ``janks`` ``janks_pc``, and
+   ``missed_vsync`` metrics.