Re: [SocketCan] Problem of accuracy

[Wolfgang Grandegger <wg@grandegger.com>]

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 µs)
> !

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.