srd: Performance improvements for various PDs.

Ignore/skip identical samples in most (low-level) PDs, as we're usually
(but not necessarily always) only interested in pin changes.

This yields a significant performance improvement for the PDs.

The mechanism was already used in the 'i2s', 'jtag', and 'lpc' PDs, but not
yet in all supported low-level decoders. The following PDs now also use
this mechanism: 'dcf77', 'i2c', 'spi', 'uart', and 'usb_signalling'.

Thanks Lars-Peter Clausen <lars@metafoo.de> for bringing this to our
attention.

1 files changed, 7 insertions, 1 deletions

diff --git a/decoders/usb_signalling/usb_signalling.py b/decoders/usb_signalling/usb_signalling.py
index 512703d..3272cbf 100644
--- a/decoders/usb_signalling/usb_signalling.py
+++ b/decoders/usb_signalling/usb_signalling.py
@@ -67,6 +67,7 @@ class Decoder(srd.Decoder):
         self.scount = 0
         self.packet = ''
         self.syms = []
+        self.oldpins = None
 
     def start(self, metadata):
         self.samplerate = metadata['samplerate']
@@ -77,13 +78,18 @@ class Decoder(srd.Decoder):
         pass
 
     def decode(self, ss, es, data):
-        for (self.samplenum, (dp, dm)) in data:
+        for (self.samplenum, pins) in data:
 
             # Note: self.samplenum is the absolute sample number, whereas
             # self.scount only counts the number of samples since the
             # last change in the D+/D- lines.
             self.scount += 1
 
+            # Ignore identical samples early on (for performance reasons).
+            if self.oldpins == pins:
+                continue
+            self.oldpins, (dp, dm) = pins, pins
+
             if self.options['signalling'] == 'low-speed':
                 sym = symbols_ls[dp, dm]
             elif self.options['signalling'] == 'full-speed':