----- Original Message ----- > From: "Finn Thain" <[email protected]> > To: "James E.J. Bottomley" <[email protected]>, "Martin K. Petersen" > <[email protected]>, "Geert > Uytterhoeven" <[email protected]>, [email protected], > [email protected], > [email protected], "Michael Schmitz" <[email protected]> > Sent: Wednesday, August 31, 2016 12:44:56 AM > Subject: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO > tranfers > > 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 reduces the PIO chunk size to reduce interrupt latency to > something on the order of milliseconds, at the expense of additional CPU > overhead from extra iterations of the NCR5380_main() loop. > > That CPU overhead is a problem for slow machines (e.g. mac_scsi on 25 MHz > 68030) but these machines generally use PDMA not PIO. This patch doesn't > make the overhead any worse on my Mac LC III (because it only gets about > 510 accesses per ms). > > This patch decreases disk performance by a fraction of one percent for > dmx3191d on my 333 MHz PowerPC 750. Other affected hardware (such as > g_NCR5380 on x86) was not tested but 5380 ISA cards generally use PDMA > and not PIO. > > Signed-off-by: Finn Thain <[email protected]> > > --- > Changed since v1: > - PIO transfer chunk size is now hard-coded for simplicity. > > --- > drivers/scsi/NCR5380.c | 8 ++++---- > drivers/scsi/NCR5380.h | 2 ++ > 2 files changed, 6 insertions(+), 4 deletions(-) > > Index: linux/drivers/scsi/NCR5380.c > =================================================================== > --- linux.orig/drivers/scsi/NCR5380.c 2016-08-31 14:44:51.000000000 +1000 > +++ linux/drivers/scsi/NCR5380.c 2016-08-31 14:44:52.000000000 +1000 > @@ -1849,11 +1849,11 @@ 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); > + transfersize = > min(cmd->SCp.this_residual, > + > NCR5380_PIO_CHUNK_SIZE); > len = transfersize; > NCR5380_transfer_pio(instance, &phase, > &len, > (unsigned char > **)&cmd->SCp.ptr); > Index: linux/drivers/scsi/NCR5380.h > =================================================================== > --- linux.orig/drivers/scsi/NCR5380.h 2016-08-31 14:44:51.000000000 +1000 > +++ linux/drivers/scsi/NCR5380.h 2016-08-31 14:44:52.000000000 +1000 > @@ -250,6 +250,8 @@ struct NCR5380_cmd { > > #define NCR5380_CMD_SIZE (sizeof(struct NCR5380_cmd)) > > +#define NCR5380_PIO_CHUNK_SIZE 256 > + > static inline struct scsi_cmnd *NCR5380_to_scmd(struct NCR5380_cmd > *ncmd_ptr) > { > return ((struct scsi_cmnd *)ncmd_ptr) - 1; > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html >
Patch makes sense, look good to me. Reviwed-by: Laurence Oberman <[email protected]>
