linux-kernel - Re: Unable to continue testing of 2.6.25

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Date:	Tue, 19 Feb 2008 11:42:46 -0800
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Frans Pop <elendil@...net.nl>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Unable to continue testing of 2.6.25

On Mon, Feb 18, 2008 at 11:18:48AM -0800, Roland Dreier wrote:
>  > > AFAIK mapping PCI memory WB is not allowed, so WC is really our only
>  > > choice.
> 
>  > afaik that depends on the BAR being prefetchable or not.
> 
> In my case the BAR is prefetchable.

Even if the BAR is prefetchable, on some platforms mapping MMIO space
as WB can cause bad results like machine check etc.

>  > (and by your argument, ioremap_cached() would not be useful, and since that was, until
>  > 2.6.25-rc1, the default behavior for ioremap(), would have caused massive problems)
> 
> I'm not sure what ioremap_cached() would really do in my case, since
> the MTRRs for PCI memory are set to UC, so without monkeying with MTRR
> contents (which can't really be done safely) the only choices we have
> are leaving the mapping as UC or using PAT to get WC.

thanks,
suresh
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