Re: ioremap_uc() followed by set_memory_wc() - burrying MTRR

[Luis R. Rodriguez]

On Wed, Apr 22, 2015 at 02:53:11PM -0400, Doug Ledford wrote:
> On Wed, 2015-04-22 at 10:17 -0600, Jason Gunthorpe wrote:
> > On Wed, Apr 22, 2015 at 05:23:28PM +0200, Luis R. Rodriguez wrote:
> > > On Tue, Apr 21, 2015 at 11:39:39PM -0600, Jason Gunthorpe wrote:
> > > > On Wed, Apr 22, 2015 at 01:39:07AM +0200, Luis R. Rodriguez wrote:
> > > > > > Mike, do you think the time is right to just remove the iPath driver?
> > > > > 
> > > > > With PAT now being default the driver effectively won't work
> > > > > with write-combining on modern kernels. Even if systems are old
> > > > > they likely had PAT support, when upgrading kernels PAT will work
> > > > > but write-combing won't on ipath.
> > > > 
> > > > Sorry, do you mean the driver already doesn't get WC? Or do you mean
> > > > after some more pending patches are applied?
> > > 
> > > No, you have to consider the system used and the effects of calls used
> > > on the driver in light of this table:
> > 
> > So, just to be clear:
> > 
> > At some point Linux started setting the PAT bits during
> > ioremap_nocache, which overrides MTRR, and at that point the driver
> > became broken on all PAT capable systems?
> > 
> > Not only that, but we've only just noticed it now, and no user ever
> > complained?
> > 
> > So that means either no users exist, or all users are on non-PAT
> > systems?
> > 
> > This driver only works on x86-64 systems. Are there any x86-64 systems
> > that are not PAT capable? IIRC even the first Opteron had PAT, but my
> > memory is fuzzy from back then :|
> > 
> > > Another option in order to enable this type of checks at run time
> > > and still be able to build the driver on standard distributions and
> > > just prevent if from loading on PAT systems is to have some code in
> > > place which would prevent the driver from loading if PAT was
> > > enabled, this would enable folks to disable PAT via a kernel command
> > > line option, and if that was used then the driver probe would
> > > complete.
> > 
> > This seems like a reasonble option to me. At the very least we might
> > learn if anyone is still using these cards.
> > 
> > I'd also love to remove the driver if it turns out there are actually
> > no users. qib substantially replaces it except for a few very old
> > cards.
> 
> To be precise, the split is that ipath powers the old HTX bus cards that
> only work in AMD systems,

Do those systems have PAT support? CAn anyone check if PAT is enabled
if booted on a recent kernel?


 Luis
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