Re: [patch] SGIIOC4 limit request size

[Jeremy Higdon <jeremy@sgi.com>]

On Wed, Feb 01, 2006 at 12:08:30PM +0100, Bartlomiej Zolnierkiewicz wrote:
> On 2/1/06, Jeremy Higdon <jeremy@sgi.com> wrote:
> > On Wed, Feb 01, 2006 at 11:34:18AM +0100, Bartlomiej Zolnierkiewicz wrote:
> > > On 01 Feb 2006 03:59:16 -0500, Jes Sorensen <jes@sgi.com> wrote:
> > > > Hi,
> > >
> > > Hi,
> > >
> > > > 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?
> >
> > Actually, it happens with a 16KB page size.
> >
> > > 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)?
> >
> > I may be misunderstanding something, but it looks to me as though
> > IOC4_PRD_ENTRIES may be ignored, since ide_init_queue() just uses
> > PRD_ENTRIES.  Fortunately, with a 16KB page size, the arithmetic
> > works out to the same.  In any case, it seems that the 64KB
> > limit is the problem.  Whether that is due to too many s/g entries
> 
> Indeed, seems that hwif->max_sg_nents is not respected when
> setting queue ->max_hw_segments and ->max_phys_segments.
> 
> Does the logic really work the same?
> Isn't PCI_DMA_BUS_IS_PHYS == 0 for SN2?
> 
> If so then the code sets ->max_{hw,phys}_segments
> to IOC4_PRD_ENTRIES/2 which actually shouldn't hurt...
> 
> > or total byte count I cannot say.  I do know that with a 2KB
> > physical sector size, the minimum size for a s/g entry should be
> > 2KB, which would mean we're using at most 32 with 127 max sectors --
> > well below the 256 that we get from PRD_ENTRIES and IOC4_PRD_ENTRIES.
> >
> > We're still looking for root cause of this problem.  But with the
> > default 128KB max request size, we occasionally get timeouts on
> > DMA commands.
> 
> I have no big problem with applying patch as it is but I think that
> it just hides the real problem and/or makes it harder to hit...
> 
> Bartlomiej

I agree.  I think I found the real problem.

I'm going to test it and sleep on it, but here is the correct patch,
I think.  Hot off the press.

Give us 16 hours  :-)

The problem was that the chip actually likes a count of 0x10000 for
a 64K s/g, but the original author programmed 0 instead of 0x10000
for that amount (I don't know why).

I'll send a better patch tomorrow.  This one depends on a byte count
multiple of 2.  Though according to the chip docs, it ignores bit 0
of the byte count anyway (and the address for that matter).  So I
think this is functionally correct.  But I think the xcount variable
is superfluous.

jeremy


--- a/linux/drivers/ide/pci/sgiioc4.c	2006-02-01 03:13:40.000000000 -0800
+++ b/linux/drivers/ide/pci/sgiioc4.c	2006-02-01 03:02:18.144450010 -0800
@@ -525,7 +525,7 @@
 				*table = 0x0;
 				table++;
 
-				xcount = bcount & 0xffff;
+				xcount = ((bcount - 1) & 0xffff) + 1;
 				*table = cpu_to_be32(xcount);
 				table++;