linux-toradex.git/drivers/net/can/dev/bittiming.c, branch v6.19

can: bittiming: add PWM validation

2025-11-26T10:20:43+00:00

Add can_validate_pwm() to validate the values pwms, pwml and pwml.
Error messages are added to each of the checks to inform the user on
what went wrong. Refer to those error messages to understand the
validation logic.

The boundary values CAN_PWM_DECODE_NS (the transceiver minimum
decoding margin) and CAN_PWM_NS_MAX (the maximum PWM symbol duration)
are hardcoded for the moment. Note that a transceiver capable of
bitrates higher than 20 Mbps may be able to handle a CAN_PWM_DECODE_NS
below 5 ns. If such transceivers become commercially available, this
code could be revisited to make this parameter configurable. For now,
leave it static.

Signed-off-by: Vincent Mailhol 
Signed-off-by: Oliver Hartkopp 
Link: https://patch.msgid.link/20251126-canxl-v8-9-e7e3eb74f889@pengutronix.de
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_validate_bitrate(): report error via netlink

2023-02-06T12:57:27+00:00

Report an error to user space via netlink if the requested bit rate is
not supported by the device.

Link: https://lore.kernel.org/all/20230202110854.2318594-18-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_sjw_set_default(): use Phase Seg2 / 2 as default for SJW

2023-02-06T12:57:27+00:00

"The (Re-)Synchronization Jump Width (SJW) defines how far a
 resynchronization may move the Sample Point inside the limits defined
 by the Phase Buffer Segments to compensate for edge phase errors." [1]

In other words, this means that the SJW parameter controls the
tolerance of the CAN controller to frequency errors compared to other
CAN controllers.

If the user space does not provide an SJW parameter, the kernel
chooses a default value of 1. This has proven to be a good default
value for classic CAN controllers, but no longer for modern CAN-FD
controllers.

In the past there were CAN controllers like the sja1000 with a rather
limited range of bit timing parameters. For the standard bit rates
this results in the following bit timing parameters:

| Bit timing parameters for sja1000 with 8.000000 MHz ref clock
|                     _----+--------------=> tseg1: 1 …   16
|                    /    /     _---------=> tseg2: 1 …    8
|                   |    |     /    _-----=> sjw:   1 …    4
|                   |    |    |    /    _-=> brp:   1 …   64 (inc: 1)
|                   |    |    |   |    /
|  nominal          |    |    |   |   |     real  Bitrt    nom   real   SampP
|  Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP  Bitrate  Error  SampP  SampP   Error  BTR0 BTR1
|  1000000    125   2    3    2   1   1  1000000   0.0%  75.0%  75.0%   0.0%   0x00 0x14
|   800000    125   3    4    2   1   1   800000   0.0%  80.0%  80.0%   0.0%   0x00 0x16
|   666666    125   4    4    3   1   1   666666   0.0%  80.0%  75.0%   6.2%   0x00 0x27
|   500000    125   6    7    2   1   1   500000   0.0%  87.5%  87.5%   0.0%   0x00 0x1c
|   250000    250   6    7    2   1   2   250000   0.0%  87.5%  87.5%   0.0%   0x01 0x1c
|   125000    500   6    7    2   1   4   125000   0.0%  87.5%  87.5%   0.0%   0x03 0x1c
|   100000    625   6    7    2   1   5   100000   0.0%  87.5%  87.5%   0.0%   0x04 0x1c
|    83333    750   6    7    2   1   6    83333   0.0%  87.5%  87.5%   0.0%   0x05 0x1c
|    50000   1250   6    7    2   1  10    50000   0.0%  87.5%  87.5%   0.0%   0x09 0x1c
|    33333   1875   6    7    2   1  15    33333   0.0%  87.5%  87.5%   0.0%   0x0e 0x1c
|    20000   3125   6    7    2   1  25    20000   0.0%  87.5%  87.5%   0.0%   0x18 0x1c
|    10000   6250   6    7    2   1  50    10000   0.0%  87.5%  87.5%   0.0%   0x31 0x1c

The attentive reader will notice that the SJW is 1 in most cases,
while the Seg2 phase is 2. Both values are given in TQ units, which in
turn is a duration in nanoseconds.

For example the 500 kbit/s configuration:

|  nominal                                  real  Bitrt    nom   real   SampP
|  Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP  Bitrate  Error  SampP  SampP   Error  BTR0 BTR1
|   500000    125   6    7    2   1   1   500000   0.0%  87.5%  87.5%   0.0%   0x00 0x1c

the TQ is 125ns, the Phase Seg2 is "2" (== 250ns), the SJW is "1" (==
125 ns).

Looking at a more modern CAN controller like a mcp2518fd, it has wider
bit timing registers.

| Bit timing parameters for mcp251xfd with 40.000000 MHz ref clock
|                     _----+--------------=> tseg1: 2 …  256
|                    /    /     _---------=> tseg2: 1 …  128
|                   |    |     /    _-----=> sjw:   1 …  128
|                   |    |    |    /    _-=> brp:   1 …  256 (inc: 1)
|                   |    |    |   |    /
|  nominal          |    |    |   |   |     real  Bitrt    nom   real   SampP
|  Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP  Bitrate  Error  SampP  SampP   Error      NBTCFG
|   500000     25  34   35   10   1   1   500000   0.0%  87.5%  87.5%   0.0%   0x00440900

The TQ is 25ns, the Phase Seg 2 is "10" (== 250ns), the SJW is "1" (==
25ns).

Since the kernel chooses a default SJW of 1 regardless of the TQ, this
leads to a much smaller SJW and thus much smaller tolerances to
frequency errors.

To maintain the same oscillator tolerances on controllers with wide
bit timing registers, select a default SJW value of Phase Seg2 / 2
unless Phase Seg 1 is less. This results in the following bit timing
parameters:

| Bit timing parameters for mcp251xfd with 40.000000 MHz ref clock
|                     _----+--------------=> tseg1: 2 …  256
|                    /    /     _---------=> tseg2: 1 …  128
|                   |    |     /    _-----=> sjw:   1 …  128
|                   |    |    |    /    _-=> brp:   1 …  256 (inc: 1)
|                   |    |    |   |    /
|  nominal          |    |    |   |   |     real  Bitrt    nom   real   SampP
|  Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP  Bitrate  Error  SampP  SampP   Error      NBTCFG
|   500000     25  34   35   10   5   1   500000   0.0%  87.5%  87.5%   0.0%   0x00440904

The TQ is 25ns, the Phase Seg 2 is "10" (== 250ns), the SJW is "5" (==
125ns). Which is the same as on the sja1000 controller.

[1] http://web.archive.org/http://www.oertel-halle.de/files/cia99paper.pdf

Link: https://lore.kernel.org/all/20230202110854.2318594-15-mkl@pengutronix.de
Cc: Mark Bath 
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_sjw_check(): check that SJW is not longer than either Phase Buffer Segment

2023-02-06T12:57:27+00:00

According to "The Configuration of the CAN Bit Timing" [1] the SJW
"may not be longer than either Phase Buffer Segment".

Check SJW against length of both Phase buffers. In case the SJW is
greater, report an error via netlink to user space and bail out.

[1] http://web.archive.org/http://www.oertel-halle.de/files/cia99paper.pdf

Link: https://lore.kernel.org/all/20230202110854.2318594-14-mkl@pengutronix.de
Suggested-by: Vincent Mailhol 
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_sjw_check(): report error via netlink and harmonize error value

2023-02-06T12:57:26+00:00

If the user space has supplied an invalid SJW value (greater than the
maximum SJW value), report -EINVAL instead of -ERANGE, this better
matches the actual meaning of the error value.

Additionally report an error message via netlink to the user space.

Link: https://lore.kernel.org/all/20230202110854.2318594-13-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_fixup_bittiming(): report error via netlink and harmonize error value

2023-02-06T12:57:26+00:00

Check each bit timing parameter first individually against their
limits and report a meaningful error message via netlink to the user
space.

In case of an error, return -EINVAL instead of -ERANGE, this
corresponds better to the actual meaning of the error value.

Link: https://lore.kernel.org/all/20230202110854.2318594-12-mkl@pengutronix.de
Suggested-by: Vincent Mailhol 
Signed-off-by: Marc Kleine-Budde

can: bittiming: factor out can_sjw_set_default() and can_sjw_check()

2023-02-06T12:57:26+00:00

Factor out the functionality of assigning a SJW default value into
can_sjw_set_default() and the checking the SJW limits into
can_sjw_check().

This functions will be improved and called from a different function
in the following patches.

Link: https://lore.kernel.org/all/20230202110854.2318594-11-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_changelink() pass extack down callstack

2023-02-06T12:57:26+00:00

This is a preparation patch.

In order to pass warning/error messages during netlink calls back to
user space, pass the extack struct down the callstack of
can_changelink(), the actual error messages will be added in the
following ptaches.

Link: https://lore.kernel.org/all/20230202110854.2318594-10-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_get_bittiming(): use direct return and remove unneeded else

2023-02-06T12:57:25+00:00

Clean up the code flow a bit, don't assign err variable but directly
return. Remove the unneeded else, too.

Link: https://lore.kernel.org/all/20230202110854.2318594-5-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde

can: bittiming: can_fixup_bittiming(): set effective tq

2023-02-06T12:57:25+00:00

The can_fixup_bittiming() function is used to validate the
user-supplied low-level bit timing parameters and calculate the
bitrate prescaler (brp) from the requested time quanta (tq) and the
CAN clock of the controller.

can_fixup_bittiming() selects the best matching integer bit rate
prescaler, which may result in a different time quantum than the value
specified by the user.

Calculate the resulting time quantum and assign it so that the user
sees the effective time quantum.

Link: https://lore.kernel.org/all/20230202110854.2318594-4-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde