Re: [PATCH] socket-can: bit-timing calculation

From: Marc Kleine-Budde
Date: Tue Jan 17 2012 - 09:34:13 EST

Hello,

Realmz Paranoid, a hint from David Miller:
> Nobody is seeing any of your postings because of "open list: NETWORKING
> DRIVERS" stuff in the email headers.

On 01/17/2012 11:25 AM, Marc Kleine-Budde wrote:
> On 01/17/2012 11:18 AM, realmz paranoid wrote:
> [...]
>
>>>>> This is worse because it does not find the best match. The first
>>>>> match might not be the best.
>>>>>
>>>>> I got it, because the tseg1_max and tseg2_max limit, the first found
>>> best
>>>> error will not get the best sample point on some hardware.
>>>> So I'd tweak the blackfin can code.
>>>
>>> The algorithm does *not* work well for all configs, especially when the
>>> clock frequency is not a multiple of 8 MHz. Instead of tweaking the
>>> algorithm just for your hardware it does make more sense to provide the
>>> parameters manually using "tq= ...". Also setting "sjw > 1" helps if the
>>> bit-timing is not optimal.
>
> I've an improved version (somewhere) that minimizes sample point error,
> too. Give me some minutes to find it.

Here it is. Wolfgang wrote the can-calc-bit-timing tool, which he used to generate
the table in the other mail. It's part of can-utils. I've pushed my improvement
to my repo (better-bitrate-calculation branch):

https://gitorious.org/~bet-frogger/linux-can/mkl-can-utils

Here's the patch:

From 4df04420f128ec326e536fe55fc819b893a5c9ef Mon Sep 17 00:00:00 2001
From: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
Date: Sat, 15 May 2010 18:26:03 +0200
Subject: [PATCH] can-calc-bit-timing: better sample point calculation

This patch tries to optimize the calculation of the sample point. To
understand what it does have a look at the original implementation.

If there is a combination of timing parameters where both the bitrate
and sample point error are 0 the current implementation has a pretty
good change of finding it (maybe it will always do, but I'm to lazy
to proof this).

However if the reference clock doesn't allow an optimal bitrate (this
means the bitrate error is always != 0) there might be several timing
parameter combinations having the same bitrate error. The original
implementation will allways choose the one with the highest brp. The
actual sample point error isn't taken into account.

This patch changes the algorithm to minimize the sample point error,
too. Now a brp/tseg combination is accepted as better if one of these
condition are fulfilled:
1) the bit rate error must be smaller, or
2) the bit rate error must be equal and
the sample point error must be equal or smaller

If a smaller bit rate error is found the sample point error is reset.
This ensures that we first optimize for small bit rate error and then
for small sample point errors.

The following diff shows the sample point error improvements.

--- orig.txt 2010-05-17 00:10:21.000000000 +0200
+++ new.txt 2010-05-17 00:10:05.000000000 +0200
@@ -27,9 +27,9 @@
Bit timing parameters for mscan with 33.000000 MHz ref clock
nominal real Bitrt nom real SampP
Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error BTR0 BTR1
1000000 90 3 4 3 1 3 1000000 0.0% 75.0% 72.7% 3.1% 0x02 0x26
- 800000 181 2 2 2 1 6 785714 1.8% 80.0% 71.4% 10.8% 0x05 0x13
+ 800000 90 5 5 3 1 3 785714 1.8% 80.0% 78.5% 1.9% 0x02 0x29
500000 181 4 4 2 1 6 500000 0.0% 87.5% 81.8% 6.5% 0x05 0x17
250000 333 4 5 2 1 11 250000 0.0% 87.5% 83.3% 4.8% 0x0a 0x18
125000 666 4 5 2 1 22 125000 0.0% 87.5% 83.3% 4.8% 0x15 0x18
100000 666 6 6 2 1 22 100000 0.0% 87.5% 86.6% 1.0% 0x15 0x1b
@@ -40,12 +40,12 @@
Bit timing parameters for mscan with 33.300000 MHz ref clock
nominal real Bitrt nom real SampP
Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error BTR0 BTR1
1000000 90 3 4 3 1 3 1009090 0.9% 75.0% 72.7% 3.1% 0x02 0x26
- 800000 180 2 2 2 1 6 792857 0.9% 80.0% 71.4% 10.8% 0x05 0x13
+ 800000 90 5 5 3 1 3 792857 0.9% 80.0% 78.5% 1.9% 0x02 0x29
500000 180 4 4 2 1 6 504545 0.9% 87.5% 81.8% 6.5% 0x05 0x17
- 250000 570 2 2 2 1 19 250375 0.1% 87.5% 71.4% 18.4% 0x12 0x13
- 125000 1141 2 2 2 1 38 125187 0.1% 87.5% 71.4% 18.4% 0x25 0x13
+ 250000 210 7 8 3 1 7 250375 0.1% 87.5% 84.2% 3.8% 0x06 0x2e
+ 125000 570 5 6 2 1 19 125187 0.1% 87.5% 85.7% 2.1% 0x12 0x1a
100000 1111 3 3 2 1 37 100000 0.0% 87.5% 77.7% 11.2% 0x24 0x15
50000 1111 7 7 3 1 37 50000 0.0% 87.5% 83.3% 4.8% 0x24 0x2d
20000 ***bitrate not possible***
10000 ***bitrate not possible***
@@ -53,12 +53,12 @@
Bit timing parameters for mscan with 33.333333 MHz ref clock
nominal real Bitrt nom real SampP
Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error BTR0 BTR1
1000000 90 3 4 3 1 3 1010101 1.0% 75.0% 72.7% 3.1% 0x02 0x26
- 800000 180 2 2 2 1 6 793650 0.8% 80.0% 71.4% 10.8% 0x05 0x13
+ 800000 90 5 5 3 1 3 793650 0.8% 80.0% 78.5% 1.9% 0x02 0x29
500000 180 4 4 2 1 6 505050 1.0% 87.5% 81.8% 6.5% 0x05 0x17
- 250000 570 2 2 2 1 19 250626 0.3% 87.5% 71.4% 18.4% 0x12 0x13
- 125000 1140 2 2 2 1 38 125313 0.3% 87.5% 71.4% 18.4% 0x25 0x13
+ 250000 210 7 8 3 1 7 250626 0.3% 87.5% 84.2% 3.8% 0x06 0x2e
+ 125000 570 5 6 2 1 19 125313 0.3% 87.5% 85.7% 2.1% 0x12 0x1a
100000 1110 3 3 2 1 37 100100 0.1% 87.5% 77.7% 11.2% 0x24 0x15
50000 870 8 8 6 1 29 49975 0.1% 87.5% 73.9% 15.5% 0x1c 0x5f
20000 ***bitrate not possible***
10000 ***bitrate not possible***
@@ -78,12 +78,12 @@

Bit timing parameters for at91 with 99.532800 MHz ref clock
nominal real Bitrt nom real SampP
Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error CAN_BR
-1000000 100 3 3 3 1 10 995328 0.5% 75.0% 70.0% 6.7% 0x00090222
+1000000 50 7 7 5 1 5 995328 0.5% 75.0% 75.0% 0.0% 0x00040664
800000 50 8 8 8 1 5 796262 0.5% 80.0% 68.0% 15.0% 0x00040777
- 500000 251 2 3 2 1 25 497664 0.5% 87.5% 75.0% 14.3% 0x00180121
- 250000 572 2 2 2 1 57 249455 0.2% 87.5% 71.4% 18.4% 0x00380111
+ 500000 100 8 8 3 1 10 497664 0.5% 87.5% 85.0% 2.9% 0x00090772
+ 250000 210 7 8 3 1 21 249455 0.2% 87.5% 84.2% 3.8% 0x00140672
125000 532 6 6 2 1 53 125198 0.2% 87.5% 86.6% 1.0% 0x00340551
100000 833 4 5 2 1 83 99932 0.1% 87.5% 83.3% 4.8% 0x00520341
50000 833 8 8 7 1 83 49966 0.1% 87.5% 70.8% 19.1% 0x00520776
20000 ***bitrate not possible***
@@ -92,22 +92,22 @@
Bit timing parameters for flexcan with 49.875000 MHz ref clock
nominal real Bitrt nom real SampP
Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error CAN_CTRL
1000000 100 3 3 3 1 5 997500 0.2% 75.0% 70.0% 6.7% 0x04120002
- 800000 180 2 2 2 1 9 791666 1.0% 80.0% 71.4% 10.8% 0x08090001
- 500000 200 3 4 2 1 10 498750 0.2% 87.5% 80.0% 8.6% 0x09190002
- 250000 501 2 3 2 1 25 249375 0.2% 87.5% 75.0% 14.3% 0x18110001
+ 800000 140 3 3 2 1 7 791666 1.0% 80.0% 77.7% 2.9% 0x06110002
+ 500000 100 8 8 3 1 5 498750 0.2% 87.5% 85.0% 2.9% 0x043a0007
+ 250000 200 8 8 3 1 10 249375 0.2% 87.5% 85.0% 2.9% 0x093a0007
125000 421 7 8 3 1 21 125000 0.0% 87.5% 84.2% 3.8% 0x143a0006
- 100000 1002 3 4 2 1 50 99750 0.2% 87.5% 80.0% 8.6% 0x31190002
+ 100000 501 8 8 3 1 25 99750 0.2% 87.5% 85.0% 2.9% 0x183a0007
50000 1664 4 5 2 1 83 50075 0.1% 87.5% 83.3% 4.8% 0x52210003
20000 3568 5 6 2 1 178 20014 0.1% 87.5% 85.7% 2.1% 0xb1290004
10000 4350 8 8 6 1 217 9992 0.1% 87.5% 73.9% 15.5% 0xd83d0007

Bit timing parameters for flexcan with 66.500000 MHz ref clock
nominal real Bitrt nom real SampP
Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP Bitrate Error SampP SampP Error CAN_CTRL
1000000 90 3 4 3 1 6 1007575 0.8% 75.0% 72.7% 3.1% 0x051a0002
- 800000 180 2 2 2 1 12 791666 1.0% 80.0% 71.4% 10.8% 0x0b090001
+ 800000 90 5 5 3 1 6 791666 1.0% 80.0% 78.5% 1.9% 0x05220004
500000 105 7 8 3 1 7 500000 0.0% 87.5% 84.2% 3.8% 0x063a0006
250000 285 5 6 2 1 19 250000 0.0% 87.5% 85.7% 2.1% 0x12290004
125000 571 5 6 2 1 38 125000 0.0% 87.5% 85.7% 2.1% 0x25290004
100000 526 7 8 3 1 35 100000 0.0% 87.5% 84.2% 3.8% 0x223a0006

Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
---
can-calc-bit-timing.c | 71 +++++++++++++++++++++++++++----------------------
1 files changed, 39 insertions(+), 32 deletions(-)

diff --git a/can-calc-bit-timing.c b/can-calc-bit-timing.c
index f8c04ba..27bf41d 100644
--- a/can-calc-bit-timing.c
+++ b/can-calc-bit-timing.c
@@ -1,6 +1,7 @@
/* can-calc-bit-timing.c: Calculate CAN bit timing parameters
*
* Copyright (C) 2008 Wolfgang Grandegger <wg@xxxxxxxxxxxxxx>
+ * Copyright (C) 2010 Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
*
* Derived from:
* can_baud.c - CAN baudrate calculation
@@ -372,18 +373,17 @@ static long common_bitrates[] = {
#define CAN_CALC_MAX_ERROR 50 /* in one-tenth of a percent */
static int can_update_spt(const struct can_bittiming_const *btc,
- int sampl_pt, int tseg, int *tseg1, int *tseg2)
+ unsigned int sampl_pt, unsigned int tseg,
+ unsigned int *tseg1, unsigned int *tseg2)
{
*tseg2 = tseg + 1 - (sampl_pt * (tseg + 1)) / 1000;
- if (*tseg2 < btc->tseg2_min)
- *tseg2 = btc->tseg2_min;
- if (*tseg2 > btc->tseg2_max)
- *tseg2 = btc->tseg2_max;
+ *tseg2 = clamp(*tseg2, btc->tseg2_min, btc->tseg2_max);
*tseg1 = tseg - *tseg2;
if (*tseg1 > btc->tseg1_max) {
*tseg1 = btc->tseg1_max;
*tseg2 = tseg - *tseg1;
}
+
return 1000 * (tseg + 1 - *tseg2) / (tseg + 1);
}
@@ -391,11 +391,15 @@ static int can_calc_bittiming(struct net_device *dev, struct can_bittiming *bt)
{
struct can_priv *priv = netdev_priv(dev);
const struct can_bittiming_const *btc = priv->bittiming_const;
- long rate, best_rate = 0;
- long best_error = 1000000000, error = 0;
- int best_tseg = 0, best_brp = 0, brp = 0;
- int tsegall, tseg = 0, tseg1 = 0, tseg2 = 0;
- int spt_error = 1000, spt = 0, sampl_pt;
+ long rate; /* current bitrate */
+ long rate_error; /* difference between current and target value */
+ long best_rate_error = 1000000000;
+ int spt; /* current sample point in thousandth */
+ int spt_error; /* difference between current and target value */
+ int best_spt_error = 1000;
+ int sampl_pt; /* target sample point */
+ int best_tseg = 0, best_brp = 0; /* current best values for tseg and brp */
+ unsigned int brp, tsegall, tseg, tseg1, tseg2;
u64 v64;
if (!priv->bittiming_const)
@@ -417,48 +421,51 @@ static int can_calc_bittiming(struct net_device *dev, struct can_bittiming *bt)
for (tseg = (btc->tseg1_max + btc->tseg2_max) * 2 + 1;
tseg >= (btc->tseg1_min + btc->tseg2_min) * 2; tseg--) {
tsegall = 1 + tseg / 2;
+
/* Compute all possible tseg choices (tseg=tseg1+tseg2) */
brp = priv->clock.freq / (tsegall * bt->bitrate) + tseg % 2;
+
/* chose brp step which is possible in system */
brp = (brp / btc->brp_inc) * btc->brp_inc;
if ((brp < btc->brp_min) || (brp > btc->brp_max))
continue;
+
rate = priv->clock.freq / (brp * tsegall);
- error = bt->bitrate - rate;
+ rate_error = abs((long)(bt->bitrate - rate));
+
/* tseg brp biterror */
- if (error < 0)
- error = -error;
- if (error > best_error)
+ if (rate_error > best_rate_error)
continue;
- best_error = error;
- if (error == 0) {
- spt = can_update_spt(btc, sampl_pt, tseg / 2,
- &tseg1, &tseg2);
- error = sampl_pt - spt;
- if (error < 0)
- error = -error;
- if (error > spt_error)
- continue;
- spt_error = error;
- }
+
+ /* reset sample point error if we have a better bitrate */
+ if (rate_error < best_rate_error)
+ best_spt_error = 1000;
+
+ spt = can_update_spt(btc, sampl_pt, tseg / 2, &tseg1, &tseg2);
+ spt_error = abs((long)(sampl_pt - spt));
+ if (spt_error > best_spt_error)
+ continue;
+
+ best_spt_error = spt_error;
+ best_rate_error = rate_error;
best_tseg = tseg / 2;
best_brp = brp;
- best_rate = rate;
- if (error == 0)
+
+ if (rate_error == 0 && spt_error == 0)
break;
}
- if (best_error) {
+ if (best_rate_error) {
/* Error in one-tenth of a percent */
- error = (best_error * 1000) / bt->bitrate;
- if (error > CAN_CALC_MAX_ERROR) {
+ rate_error = (best_rate_error * 1000) / bt->bitrate;
+ if (rate_error > CAN_CALC_MAX_ERROR) {
dev_err(dev->dev.parent,
"bitrate error %ld.%ld%% too high\n",
- error / 10, error % 10);
+ rate_error / 10, rate_error % 10);
return -EDOM;
} else {
dev_warn(dev->dev.parent, "bitrate error %ld.%ld%%\n",
- error / 10, error % 10);
+ rate_error / 10, rate_error % 10);
}
}
-- 1.7.4.1

--
Pengutronix e.K. | Marc Kleine-Budde |
Industrial Linux Solutions | Phone: +49-231-2826-924 |
Vertretung West/Dortmund | Fax: +49-5121-206917-5555 |
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