[Xenomai-help] Re: Latencies due to RT-Socket-CAN register accesses

[Wolfgang Grandegger <wg@domain.hid>]

Hi Roland,

Roland Tollenaar wrote:
> Hi Wolfgang,
> 
> Well these are convincing figures. Could you let me know what card (type 
> number) you ran the tests on so that I am not unpleasantly surprised 
> when I finally go to PCI.

It was the IXXAT PCI card currently plugged into my test PC but I 
actually recommand the PEAK PCI card. It's also much cheaper, I guess. 
Tomorrow I'm going to repeat the tests with this card ... stay tuned.

> I will continue development on my laptop with the dongle for the time 
> being. It would be interesting to see what happens to the latency after 
> your proposed changes are in effect.

BTW, what are the latencies you measure on your system under load 
(without RT-Socket-CAN).

Wolfgang.

> Thanks,
> 
> Roland
> 
> 
> 
> Wolfgang Grandegger wrote:
>> Hallo,
>>
>> in the meantime I have measured the latencies introduced through 
>> messages sent and received by RT-Socket-CAN. The SJA1000 register 
>> access times on my rather old PC with an Athlon 1100 Mhz are:
>>
>> PEAK-Dongle: read  access: 11807 ns
>> PEAK-Dongle: write access: 11677 ns
>>
>> IXXAT-PCI  : read  access:   729 ns
>> IXXAT-PCI  : write access:   305 ns
>>
>> I measured an increase of the latency of approx. 170us with the 
>> PEAK-Dongle and approx 13us with the IXXAT-PCI card for the reception 
>> of a full CAN message (with 8 bytes payload). Sending messages is a 
>> bit less disturbing. I have attached a small patch to measure the 
>> SJA1000 register access times when the driver is initialized. You are 
>> welcome to apply it on your setup and report the results. I'm 
>> especially interested in numbers for the ISA bus (or PC-104).
>>
>> Wolfgang.
>>
>>
>>
>> ------------------------------------------------------------------------
>>
>> + diff -u xenomai/ksrc/drivers/can/rtcan_dev.c.IOTEST 
>> xenomai/ksrc/drivers/can/rtcan_dev.c
>> + diff -u xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c.IOTEST 
>> xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c
>> --- xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c.IOTEST    
>> 2007-02-26 09:17:27.000000000 +0100
>> +++ xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c    2007-03-13 
>> 10:01:47.000000000 +0100
>> @@ -728,6 +728,30 @@
>>      if (chip == NULL)
>>      return -EINVAL;
>>  
>> +#if 1
>> +    {
>> +    nanosecs_abs_t begin, diff;
>> +    volatile u8 reg;
>> +    int i, count = 100000;
>> +    begin = rtdm_clock_read();
>> +    for (i = 0; i < count; i++) {
>> +        reg = chip->read_reg(dev, 0);
>> +    }
>> +    diff = rtdm_clock_read() - begin;
>> +    printk("%s: register read time for %d accessed: %ld (%ld per 
>> access)\n",
>> +           dev->board_name, count,
>> +           (unsigned long)diff, (unsigned long)diff / count);
>> +    begin = rtdm_clock_read();
>> +    for (i = 0; i < count; i++) {
>> +        chip->write_reg(dev, 0, reg);
>> +    }
>> +    diff = rtdm_clock_read() - begin;
>> +    printk("%s: register write time for %d accessed: %ld (%ld per 
>> access)\n",
>> +           dev->board_name, count,
>> +           (unsigned long)diff, (unsigned long)diff / count);
>> +    }
>> +#endif
>> +
>>      /* Set dummy state for following call */
>>      dev->state = CAN_STATE_ACTIVE;
>>      /* Enter reset mode */
> 
>