Re: [PATCH 2/2] cciss: disable dma prefetch for P600

On Wed, Oct 18, 2006 at 02:37:23PM -0700, Andrew Morton wrote:

argh, you removed the mailing list from cc.

Sorry, I'm still lacking proper etiquette.


On Wed, 18 Oct 2006 11:54:53 -0500
"Mike Miller (OS Dev)" <mikem@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, Oct 17, 2006 at 05:10:21PM -0700, Andrew Morton wrote:
On Tue, 17 Oct 2006 16:13:03 -0500
"Mike Miller (OS Dev)" <mikem@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

PATCH 2/2
Turned off DMA prefetch for the P600 on systems which may present
discontiguous memory.


What do you mean by "discontiguous memory"? CONFIG_DISCONTIGMEM?

The IPF memory map can have holes between the different regions. I've
been told by our HW guys that AMD may also have holes.

Pretty much all platforms/architectures have holes in their physical memory
map.



What is the actual problem which is being fixed here?

Sorry, I should have been clearer. There is a bug in the DMA engine that
that may result in prefetching data from beyond the end of memory or
falling off into one the holes on IPF and AMD. It causes a machine check
when that happens.
It doesn't happen on Proliant because the last 4kB (or so) of memory is
mapped out by the BIOS and Pentium guarantees contiguous memory.

I think that this:

#if defined(CONFIG_IA64) || defined(CONFIG_X86_64)

is nowhere near strong enough and is probably inappropriate.

It _could_ be that CONFIG_DISCONTIGMEM|CONFIG_SPARSEMEM will be closer, but
even CONFIG_FLATMEM systems can have holes.

I'm poking around on some IPF platforms. It looks like CONFIG_DISCONTIGMEM is
set on them, but not the others you mention. Would that be sufficient?


On what machines can/does this card exist? Things like powerpc?

This problem was found on Itanium. We don't try to support powerpc.

Thanks,
mikem
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