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.

1 files changed, 8 insertions, 5 deletions

diff --git a/decoders/uart/pd.py b/decoders/uart/pd.py
index d42a5d4..db1065d 100644
--- 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':