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can: add documentation for CAN filter usage optimisation

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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>
This commit is contained in:
Oliver Hartkopp 2014-04-02 20:25:27 +02:00 committed by Marc Kleine-Budde
parent 45c700291a
commit 277bd320e7
1 changed files with 35 additions and 0 deletions
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35
Documentation/networking/can.txt
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@ -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