i2cfilter: rephrase decoder implementation for maintainability

The previous implementation suffered from a severe issue (it kept
referencing caller's data beyond its validity) and from style issues
(redundant details, conditions scattered across distant locations).

Rephrase (actually rework) the decoder implementation to improve
readability as well as maintainability. Extend the TODO list while
the existing logic better lends itself to future extensions. Reword
comments while we are here. Some earlier constraints no longer apply
or were unfortunately phrased.

diff --git a/decoders/i2cfilter/pd.py b/decoders/i2cfilter/pd.py
index a54baab247b88a474db2c08a3649d097ba2c5e52..877c4676a0fe895679dab96c00cac862dff4fdd1 100644 (file)
--- a/decoders/i2cfilter/pd.py
+++ b/decoders/i2cfilter/pd.py
@@ -18,8 +18,12 @@
 ## along with this program; if not, see <http://www.gnu.org/licenses/>.
 ##
 
-# TODO: Support for filtering out multiple slave/direction pairs?
+# TODO
+# - Accept other slave address forms than decimal numbers?
+# - Support for filtering out multiple slave/direction pairs?
+# - Support 10bit slave addresses?
 
+import copy
 import sigrokdecode as srd
 
 class Decoder(srd.Decoder):
@@ -43,51 +47,60 @@ class Decoder(srd.Decoder):
         self.reset()
 
     def reset(self):
-        self.curslave = -1
-        self.curdirection = None
-        self.packets = [] # Local cache of I²C packets
+        self.seen_packets = []
+        self.do_forward = None
 
     def start(self):
         self.out_python = self.register(srd.OUTPUT_PYTHON, proto_id='i2c')
         if self.options['address'] not in range(0, 127 + 1):
             raise Exception('Invalid slave (must be 0..127).')
+        self.want_addrs = []
+        if self.options['address']:
+            self.want_addrs.append(self.options['address'])
+        self.want_dir = {
+            'read': 'READ', 'write': 'WRITE',
+        }.get(self.options['direction'], None)
 
-    # Grab I²C packets into a local cache, until an I²C STOP condition
-    # packet comes along. At some point before that STOP condition, there
-    # will have been an ADDRESS READ or ADDRESS WRITE which contains the
-    # I²C address of the slave that the master wants to talk to.
-    # If that slave shall be filtered, output the cache (all packets from
-    # START to STOP) as proto 'i2c', otherwise drop it.
-    def decode(self, ss, es, data):
+    def _need_to_forward(self, slave_addr, direction):
+        if self.want_addrs and slave_addr not in self.want_addrs:
+            return False
+        if self.want_dir and direction != self.want_dir:
+            return False
+        return True
 
-        cmd, databyte = data
+    # Accumulate observed I2C packets until a STOP or REPEATED START
+    # condition is seen. These are conditions where transfers end or
+    # where direction potentially changes. Forward all previously
+    # accumulated traffic if it passes the slave address and direction
+    # filter. This assumes that the slave address as well as the read
+    # or write direction was part of the observed traffic. There should
+    # be no surprise when incomplete traffic does not match the filter
+    # condition.
+    def decode(self, ss, es, data):
 
-        # Add the I²C packet to our local cache.
-        self.packets.append([ss, es, data])
+        # Unconditionally accumulate every lower layer packet we see.
+        # Keep deep copies for later, only reference caller's values
+        # as long as this .decode() invocation executes.
+        self.seen_packets.append([ss, es, copy.deepcopy(data)])
+        cmd, _ = data
 
+        # Check the slave address and transfer direction early when
+        # we see them. Keep accumulating packets while it's already
+        # known here whether to forward them. This simplifies other
+        # code paths. Including future handling of 10bit addresses.
         if cmd in ('ADDRESS READ', 'ADDRESS WRITE'):
-            self.curslave = databyte
-            self.curdirection = cmd[8:].lower()
-        elif cmd in ('STOP', 'START REPEAT'):
-            # If this chunk was not for the correct slave, drop it.
-            if self.options['address'] == 0:
-                pass
-            elif self.curslave != self.options['address']:
-                self.packets = []
-                return
-
-            # If this chunk was not in the right direction, drop it.
-            if self.options['direction'] == 'both':
-                pass
-            elif self.options['direction'] != self.curdirection:
-                self.packets = []
-                return
-
-            # TODO: START->STOP chunks with both read and write (Repeat START)
-            # Otherwise, send out the whole chunk of I²C packets.
-            for p in self.packets:
-                self.put(p[0], p[1], self.out_python, p[2])
+            direction = cmd[len('ADDRESS '):]
+            _, slave_addr = data
+            self.do_forward = self._need_to_forward(slave_addr, direction)
+            return
 
-            self.packets = []
-        else:
-            pass # Do nothing, only add the I²C packet to our cache.
+        # Forward previously accumulated packets as we see their
+        # completion, and when they pass the filter condition. Prepare
+        # to handle the next transfer (the next read/write part of it).
+        if cmd in ('STOP', 'START REPEAT'):
+            if self.do_forward:
+                for ss, es, data in self.seen_packets:
+                    self.put(ss, es, self.out_python, data)
+            self.seen_packets.clear()
+            self.do_forward = None
+            return