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canbusload: Fix worst-case frame length estimation 

The Tindell's method used previously is incorrect. It does not account for
the fact that the stuffed bits are themselves also subject to bit stuffing.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
pull/1/head
Michal Sojka 2014-01-30 14:38:59 +01:00 committed by Marc Kleine-Budde
parent 2206f92de7
commit d6b576c012
2 changed files with 11 additions and 8 deletions
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2
canframelen.c
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@ -246,7 +246,7 @@ unsigned can_frame_length(struct canfd_frame *frame, enum cfl_mode mode, int mtu
case CFL_NO_BITSTUFFING:
return (eff ? 67 : 47) + frame->len * 8;
case CFL_WORSTCASE:
return ((eff ? 389 : 269) + frame->len * 48) / 5;
return (eff ? 80 : 55) + frame->len * 10;
case CFL_EXACT:
return cfl_exact((struct can_frame*)frame);
}

17
canframelen.h
View File

@ -6,20 +6,23 @@
/**
* Frame length calculation modes.
*
* CFL_WORSTCASE corresponds to Ken Tindells *worst* case calculation
* for stuff-bits (see "Guaranteeing Message Latencies on Controller
* Area Network" 1st ICC'94) the needed bits on the wire can be
* calculated as:
* CFL_WORSTCASE corresponds to *worst* case calculation for
* stuff-bits - see (1)-(3) in [1]. The worst case number of bits on
* the wire can be calculated as:
*
* (34 + 8n)/5 + 47 + 8n for SFF frames (11 bit CAN-ID) => (269 + 48n)/5
* (54 + 8n)/5 + 67 + 8n for EFF frames (29 bit CAN-ID) => (389 + 48n)/5
* (34 + 8n - 1)/4 + 34 + 8n + 13 for SFF frames (11 bit CAN-ID) => 55 + 10n
* (54 + 8n - 1)/4 + 54 + 8n + 13 for EFF frames (29 bit CAN-ID) => 80 + 10n
*
* while 'n' is the data length code (number of payload bytes)
*
* [1] "Controller Area Network (CAN) schedulability analysis:
* Refuted, revisited and revised", Real-Time Syst (2007)
* 35:239-272.
*
*/
enum cfl_mode {
CFL_NO_BITSTUFFING, /* plain bit calculation without bitstuffing */
CFL_WORSTCASE, /* with bitstuffing following Tindells estimation */
CFL_WORSTCASE, /* worst case estimation - see above */
CFL_EXACT, /* exact calculation of stuffed bits based on frame
* content and CRC */
};