Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers

Martin K. Petersen
2016-09-09T11:29:05Z

>>>>> "Finn" == Finn Thain <fthain@telegraphics.com.au> writes:

Finn> Large PIO transfers are broken up into chunks to try to avoid
Finn> disabling local IRQs for long periods. But IRQs are still disabled
Finn> for too long and this causes SCC FIFO overruns during serial port
Finn> transfers.

Finn> This patch reduces the PIO chunk size to reduce interrupt latency
Finn> to something on the order of milliseconds, at the expense of
Finn> additional CPU overhead from extra iterations of the
Finn> NCR5380_main() loop.

Finn> That CPU overhead is a problem for slow machines (e.g. mac_scsi on
Finn> 25 MHz
Finn> 68030) but these machines generally use PDMA not PIO. This patch
Finn> doesn't
Finn> make the overhead any worse on my Mac LC III (because it only gets
Finn> about 510 accesses per ms).

Finn> This patch decreases disk performance by a fraction of one percent
Finn> for dmx3191d on my 333 MHz PowerPC 750. Other affected hardware
Finn> (such as g_NCR5380 on x86) was not tested but 5380 ISA cards
Finn> generally use PDMA and not PIO.

Geert?

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Martin K. Petersen Oracle Linux Engineering