From 277bd320e7dd8b77de6d3b6b9757b08fcca0108b Mon Sep 17 00:00:00 2001 From: Oliver Hartkopp Date: Wed, 2 Apr 2014 20:25:27 +0200 Subject: [PATCH] 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 Signed-off-by: Marc Kleine-Budde --- Documentation/networking/can.txt | 35 ++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) 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 -- 2.20.1