linux-kernel - Re: [PATCH v2 3/6] x86, mm, asm-gen: Add ioremap

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Message-ID: <1410378027.28990.268.camel@misato.fc.hp.com>
Date:	Wed, 10 Sep 2014 13:40:27 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juergen Gross <jgross@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Yigal Korman <yigal@...xistor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2 3/6] x86, mm, asm-gen: Add ioremap_wt() for WT

On Wed, 2014-09-10 at 11:29 -0700, Andy Lutomirski wrote:
> On Wed, Sep 10, 2014 at 9:51 AM, Toshi Kani <toshi.kani@...com> wrote:
 :
> > +#ifndef ARCH_HAS_IOREMAP_WT
> > +#define ioremap_wt ioremap_nocache
> > +#endif
> > +
> 
> This is a little bit sad.  I wouldn't be too surprised if there are
> eventually users who prefer WC or WB over UC if WT isn't available
> (and they'll want a corresponding way to figure out what kind of fence
> to use).

Right, this redirection is not ideal for the performance, but it is done
this way for the correctness.  WT & UC have strongly ordered writes, but
WB & WC do not.

Thanks,
-Toshi

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