Re: [SocketCan] Problem of accuracy

[Wolfgang Grandegger <wg@grandegger.com>]

On 07/08/2012 10:30 PM, Mohamed HAMZAOUI wrote:
> The other transeiver is a KVASER USBCan II (HS,HS) it's an usb gateway
> CAN and it's accurate because i use it with other CAN transeiver
> (IXXAT PCI or Vector CanboardXL) and i have always the same result.

Well, I would not use a USB CAN controller for accurate timing (if it
does not use hardware time stamps). Anyway, I think that's not the
problem, as Kurt already pointed out.

> My board is Craneboard "AM3517-crane" and i compiled my system with
> OpenEmbedded which done : (cat /proc/version) Linux version 2.6.32
> (requinham@pearl) (gcc version 4.5.3 20110311 (prerelease) (GCC) ) #1
> PREEMPT Fri Jul 6 21:46:07 CEST 2012

OK, then you use the ti_hecc driver, which hit mainline in 2.6.33! Could
you show as the source of your "drivers/net/can/ti_hecc.c"? There is at
least one issue in the early driver version, which makes sense to fix:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=e3f240f460a36b158217944b52a85f304914c1a6

If that doesn't help, you should use ftrace to understand what's going
on. Well, not sure if ftrace already works with 2.6.32 on ARM... that
would be another good reason to update to a more recent version of the
kernel. Would that be an option for you?

Wolfgang.

> On Sun, Jul 8, 2012 at 10:13 PM, Wolfgang Grandegger <wg@grandegger.com> wrote:
>> On 07/08/2012 09:43 PM, Mohamed HAMZAOUI wrote:
>>> Hi,
>>>
>>>> This problem is not CAN related. Anyway, you can read/try/check:
>>>>
>>>> - are high-resolution timers enabled in your kernel?
>>>
>>> My high resolution timers is enabled in Kernel and i activate
>>> Preemptible kernel (low latency desktop)
>>>
>>>> - use cyclictest [1] to measure user-space timing accuracy.
>>>
>>> the cyclictest mesure with 0% load is 307 in maximum
>>
>> You mean 302 us? Did you generate some load? That normally makes a big
>> difference.
>>
>>>
>>>> - use clock_nanosleep for periodic tasks.
>>>
>>> i use clock_nanosleep with (1000000 ns = 1ms) and the result mesured
>>> with another CAN transeiver is an average of 1.30 ms (delta = 300 盜)
>>> !
>>
>> That means that you see a period of 1.3ms instead of 1ms? That's really
>> strange. I assume that the timing on the system with the other CAN
>> transceiver is accurate.
>>
>>>> - try to start cangen with "chrt -f 10 cangen can0".
>>>
>>> no effect.
>>>
>>>> - read "man sched_setscheduler" for a description of the Linux
>>>>   scheduling scheme.
>>>
>>> I test with two type of SCHED : RR and FIFO but the result is same
>>>
>>> Maybe i need to pass to Xenomai or PREEMPT-RT ?
>>
>> What system, kernel and CAN controller do you use? Is it an MCP25xx?
>>
>> Wolfgang.
>>
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