uart: Optimize handling of samples when tx and rx are both idle

Re-enable the fast path for identical samples but only when both
pins are waiting for the start bit. For sparse data sets (I tested
UT61E capture log) the optimization results in a >4x decode
improvement.

diff --git a/decoders/uart/pd.py b/decoders/uart/pd.py
index d42a5d42a4c9a89537e4b78b773dcc1cf24db3fb..db1065dac4f42e1ec03e8f953518cd3ad435eeb4 100644 (file)
--- a/decoders/uart/pd.py
+++ b/decoders/uart/pd.py
@@ -138,6 +138,7 @@ class Decoder(srd.Decoder):
         ('tx', 'TX dump'),
         ('rxtx', 'RX/TX dump'),
     )
+    idle_state = ['WAIT FOR START BIT', 'WAIT FOR START BIT']
 
     def putx(self, rxtx, data):
         s, halfbit = self.startsample[rxtx], self.bit_width / 2.0
@@ -171,7 +172,7 @@ class Decoder(srd.Decoder):
         self.startsample = [-1, -1]
         self.state = ['WAIT FOR START BIT', 'WAIT FOR START BIT']
         self.oldbit = [1, 1]
-        self.oldpins = [1, 1]
+        self.oldpins = [-1, -1]
         self.databits = [[], []]
 
     def start(self):
@@ -338,10 +339,12 @@ class Decoder(srd.Decoder):
             raise SamplerateError('Cannot decode without samplerate.')
         for (self.samplenum, pins) in data:
 
-            # Note: Ignoring identical samples here for performance reasons
-            # is not possible for this PD, at least not in the current state.
-            # if self.oldpins == pins:
-            #     continue
+            # We want to skip identical samples for performance reasons but,
+            # for now, we can only do that when we are in the idle state
+            # (meaning both channels are waiting for the start bit).
+            if self.state == self.idle_state and self.oldpins == pins:
+                continue
+
             self.oldpins, (rx, tx) = pins, pins
 
             if self.options['invert_rx'] == 'yes':