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authorOliver Hartkopp <socketcan@hartkopp.net>2014-04-02 20:25:27 +0200
committerMarc Kleine-Budde <mkl@pengutronix.de>2014-05-19 09:38:25 +0200
commit277bd320e7dd8b77de6d3b6b9757b08fcca0108b (patch)
tree4b62b370215eae7855e29bc3b750309f9a8b9c92 /Documentation
parent45c700291aee5170185bf5d1c2a494b1e3fe0883 (diff)
can: add documentation for CAN filter usage optimisation
To benefit from special filters for single SFF or single EFF CAN identifier subscriptions the CAN_EFF_FLAG bit and the CAN_RTR_FLAG bit has to be set together with the CAN_(SFF|EFF)_MASK in can_filter.mask. Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/networking/can.txt35
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diff --git a/Documentation/networking/can.txt b/Documentation/networking/can.txt
index 2fa44cbe81b7..cdd381c5311d 100644
--- a/Documentation/networking/can.txt
+++ b/Documentation/networking/can.txt
@@ -469,6 +469,41 @@ solution for a couple of reasons:
having this 'send only' use-case we may remove the receive list in the
Kernel to save a little (really a very little!) CPU usage.
+ 4.1.1.1 CAN filter usage optimisation
+
+ The CAN filters are processed in per-device filter lists at CAN frame
+ reception time. To reduce the number of checks that need to be performed
+ while walking through the filter lists the CAN core provides an optimized
+ filter handling when the filter subscription focusses on a single CAN ID.
+
+ For the possible 2048 SFF CAN identifiers the identifier is used as an index
+ to access the corresponding subscription list without any further checks.
+ For the 2^29 possible EFF CAN identifiers a 10 bit XOR folding is used as
+ hash function to retrieve the EFF table index.
+
+ To benefit from the optimized filters for single CAN identifiers the
+ CAN_SFF_MASK or CAN_EFF_MASK have to be set into can_filter.mask together
+ with set CAN_EFF_FLAG and CAN_RTR_FLAG bits. A set CAN_EFF_FLAG bit in the
+ can_filter.mask makes clear that it matters whether a SFF or EFF CAN ID is
+ subscribed. E.g. in the example from above
+
+ rfilter[0].can_id = 0x123;
+ rfilter[0].can_mask = CAN_SFF_MASK;
+
+ both SFF frames with CAN ID 0x123 and EFF frames with 0xXXXXX123 can pass.
+
+ To filter for only 0x123 (SFF) and 0x12345678 (EFF) CAN identifiers the
+ filter has to be defined in this way to benefit from the optimized filters:
+
+ struct can_filter rfilter[2];
+
+ rfilter[0].can_id = 0x123;
+ rfilter[0].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_SFF_MASK);
+ rfilter[1].can_id = 0x12345678 | CAN_EFF_FLAG;
+ rfilter[1].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_EFF_MASK);
+
+ setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
+
4.1.2 RAW socket option CAN_RAW_ERR_FILTER
As described in chapter 3.4 the CAN interface driver can generate so