Re: [PATCH 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers

Geert Uytterhoeven
2016-08-28T08:08:47Z

Hi Finn,

On Sat, Aug 27, 2016 at 4:30 AM, Finn Thain <fthain@telegraphics.com.au> wrote:
> Large PIO transfers are broken up into chunks to try to avoid disabling
> local IRQs for long periods. But IRQs are still disabled for too long
> and this causes SCC FIFO overruns during serial port transfers. This
> patch fixes the problem by halving the PIO chunk size.
>
> Testing with mac_scsi shows that the extra NCR5380_main() loop iterations
> have negligible performance impact on SCSI transfers (about 1% slower).
> On a faster system (using the dmx3191d module) transfers showed no
> measurable change.
>
> Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
>
> ---
> drivers/scsi/NCR5380.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> Index: linux/drivers/scsi/NCR5380.c
> ===================================================================
> --- linux.orig/drivers/scsi/NCR5380.c 2016-08-27 12:29:57.000000000 +1000
> +++ linux/drivers/scsi/NCR5380.c 2016-08-27 12:29:58.000000000 +1000
> @@ -1847,11 +1847,11 @@ static void NCR5380_information_transfer
> /* XXX - need to source or sink data here, as appropriate */
> }
> } else {
> - /* Break up transfer into 3 ms chunks,
> - * presuming 6 accesses per handshake.
> + /* Transfer a small chunk so that the
> + * irq mode lock is not held too long.
> */
> transfersize = min((unsigned long)cmd->SCp.this_residual,
> - hostdata->accesses_per_ms / 2);
> + hostdata->accesses_per_ms >> 2);

I think it's easier to read if you use "/ 4".

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds