Re: [patch] SGIIOC4 limit request size

[Jes Sorensen <jes@sgi.com>]

Bartlomiej Zolnierkiewicz wrote:
> On 01 Feb 2006 03:59:16 -0500, Jes Sorensen <jes@sgi.com> wrote:
> > This one takes care of a problem with the SGI IOC4 driver where it
> > hits DMA problems if the request grows too large.
>
>
> Does this happen only for CONFIG_IA64_PAGE_SIZE_4KB=y
> or CONFIG_IA64_PAGE_SIZE_8KB=y?
>
> from sgiioc4.c:
>
> /* Each Physical Region Descriptor Entry size is 16 bytes (2 * 64 bits) */
> /* IOC4 has only 1 IDE channel */
> #define IOC4_PRD_BYTES       16
> #define IOC4_PRD_ENTRIES     (PAGE_SIZE /(4*IOC4_PRD_BYTES))
>
> As limiting request size to 127 sectors punishes performance
> wouldn't it be better to define IOC4_PRD_ENTRIES to 256
> if this is possible (would need 4 pages for PAGE_SIZE=4096
> and 2 for PAGE_SIZE=8192)?

This happens with the default page size which is 16KB, ie.
IOC4_PRD_ENTRIES=256, the problem is not due to the request
going beyond the number of PRD_ENTRIES. I haven't tried with smaller
page sizes but I would assume the problem would be the same.

Even with this patch performance seems very reasonable.

Jes
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