Date: Thu, 10 Jan 2008 11:02:54 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Dave Airlie <airlied@gmail.com>
Cc: Andi Kleen <ak@suse.de>, linux-kernel@vger.kernel.org,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
       Venki Pallipadi <venkatesh.pallipadi@intel.com>,
       suresh.b.siddha@intel.com, Arjan van de Ven <arjan@infradead.org>,
       Dave Jones <davej@redhat.com>
Subject: Re: CPA patchset
Message-ID: <20080110100254.GA28209@elte.hu>
References: <20080103424.989432000@suse.de> <20080110093126.GA360@elte.hu> <21d7e9970801100144x63b02e1bga548acd3c5bed441@mail.gmail.com>
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* Dave Airlie <airlied@gmail.com> wrote:

> > - firstly, there's no rationale given. So we'll change ioremap()/etc.
> >   from doing a cflush-range instruction instead of a WBINVD. But why?
> >   WBINVD isnt particular fast (takes a few msecs), but why is that a
> >   problem? Drivers dont do high-frequency ioremap-ing. It's typically
> >   only done at driver/device startup and that's it. Whether module load
> >   time takes 1254 msecs instead of 1250 msecs is no big deal.
> 
> read graphics drivers, even though I think we may avoid the whole path 
> if we can and end up doing some of this in the drivers when they know 
> more about the situation so can avoid safeties..

by 'read graphics drivers' do you mean direct framebuffer access? In any 
case, a driver (or even userspace) can use cflush just fine, it's an 
unprivileged instruction. But this is about the ioremap() implementation 
using cflush instead of WBINVD, and that is a slowpath and is up to the 
kernel anyway - i'm not aware of many high-frequency ioremap() users, so 
robustness concerns control the policy here. So could you please explain 
your point in more detail?

	Ingo
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