linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Date:	Wed, 21 Oct 2009 16:38:06 +0200
From:	Thomas Schlichter <thomas.schlichter@....de>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	dri-devel@...ts.sourceforge.net,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"H. Peter Anvin" <hpa@...or.com>, jbarnes@...tuousgeek.org,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Ingo Molnar <mingo@...hat.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Tejun Heo <tj@...nel.org>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	x86@...nel.org, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not  available

Jan Beulich wrote:
> >Yes, I had that in the first place, but Jan suggested to extend it to also
> >handle non-aligned, non-power-of-two cases:
> >  http://marc.info/?l=linux-kernel&m=125541951529918&w=2 
> 
> I merely pointed out it wouldn't work for unaligned addresses or sizes
> passed in.

Oh, I'm sorry, I must have misinterpreted it...

> >So if it's OK for Jan, I'll reduce the functionality again and use mtrr_add()
> >instead. Btw. this simply means to drop mtrr_add_unaligned(), all the other
> >parts are still required for reference counting and a proper mtrr_del() on file
> >close.
> 
> I'm perfectly fine with just handling the aligned case, as long as the code
> checks that the alignment constraints are met.

Hmm, as far as I see mtrr_add() and mtrr_add_page() already check these
constraints. Do you want me to check them additionally? Or do you want to
completely fail the mmap() if these constraints are violated?

I'd let mmap() succeed even if the mtrr_add() fails...

Regards,
  Thomas
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