linux-kernel - Re: [RFC PATCH 02/12] PAT 64b: Basic PAT implementation

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Date:	Fri, 14 Dec 2007 13:06:45 -0800
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	venkatesh.pallipadi@...el.com, ak@....de, rdreier@...co.com,
	torvalds@...ux-foundation.org, gregkh@...e.de, airlied@...net.ie,
	davej@...hat.com, mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, arjan@...radead.org,
	jesse.barnes@...el.com, linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [RFC PATCH 02/12] PAT 64b: Basic PAT implementation

On Thu, Dec 13, 2007 at 08:48:45PM -0700, Eric W. Biederman wrote:
> > +		pat = PAT(0,WB) | PAT(1,WT) | PAT(2,UC_MINUS) | PAT(3,WC) |
> > +		      PAT(4,WB) | PAT(5,WT) | PAT(6,UC_MINUS) | PAT(7,WC);
> 
> I strongly object to this configuration.
> 
> The caching modes of interest are:
> PAT_WB write-back or a close as the MTRRs will allow
>        used for WC today.
> PAT_UC completely uncachable not overridable by MTRRs 
>        and what we use today for pgprot_noncached
> PAT_WC what isn't available for current use.
>
> We should use:
> > +		pat = PAT(0,WB) | PAT(1,WT) | PAT(2,WC) | PAT(3,UC) |
> > +		      PAT(4,WB) | PAT(5,WT) | PAT(6,WC) | PAT(7,UC);
> 
> Changing the UC- which currently allows write-combining if the MTRRs specify it,
> to WC.  This grandfathers in all of our current usage and changes the one
> PAT type that could today and in legacy mode specify WC to really specify WC.

That seems reasonable. But looking at mainline kernel, ioremap_nocache()
actually uses UC_MINUS. Wonder why it is not using UC (like
pgprot_noncached).  I think it is ok to change ioremap_nocache() to use UC.

thanks,
suresh
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