Subject: Re: Writeback cache issues
To: Matt Thomas <firstname.lastname@example.org>
From: Per Fogelstrom <email@example.com>
Date: 08/28/1995 17:07:47
> In <199508280906.LAA04574@outside.enea.se> , you wrote:
> > Have anyone had a look at those nasty 'whatchouts' that one
> > get with write-back caches and 'direct' dma?
> That really depends on the architecture of the system. Alpha
> and most Intel platforms perform coherent DMA so this isn't an
> issue. MIPS (port pmax) does not have coherent DMA and so the
> driver is responsible for using either non-cached memory or is
> responsible for making sure the cache is flushed on DMA operations.
It is no problem to flush caches when one knows a dma operation is
coming up. And right, systems with working cache snooping doesn't
have to bother. So the above is not a 'problem'. And pmaxe's has,
from what i know, write through caches.
> > What i think about is, for instance, the small data structures
> > used in the scsi drivers for reading sense data etc. There is
> > a potential problem with these structures if they share cache
> > lines with data used by the driver or other code.
> If the are small, then the best solution might to allocate them
> out of non-cached memory (for those system without coherent
> DMA implementations).
You are probably right here. Even if it doesn't have to be uncached
but aligned to cache lines. But that's up to the driver. Hmm..