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

[Roland Tollenaar <rwatollenaar@domain.hid>]

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.

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.

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 */