linux-kernel - Re: [PATCH 1/4] AMD IOMMU: use status bit instead of memory write-back for completion wait

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Date:	Thu, 14 Aug 2008 22:02:47 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Ingo Oeser <ioe-lkml@...eria.de>
Cc:	Joerg Roedel <joerg.roedel@....com>, mingo@...hat.com,
	tglx@...utronix.de, hpa@...or.com, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 1/4] AMD IOMMU: use status bit instead of memory write-back for completion wait

On Thu, Aug 14, 2008 at 08:35:30PM +0200, Ingo Oeser wrote:
> Hi Joerg,
> 
> On Thursday 14 August 2008, Joerg Roedel wrote:
> > diff --git a/arch/x86/kernel/amd_iommu.c b/arch/x86/kernel/amd_iommu.c
> > index 22d7d05..028e945 100644
> > --- a/arch/x86/kernel/amd_iommu.c
> > +++ b/arch/x86/kernel/amd_iommu.c
> > @@ -122,9 +119,15 @@ static int iommu_completion_wait(struct amd_iommu *iommu)
> >  
> >  	while (!ready && (i < EXIT_LOOP_COUNT)) {
> >  		++i;
> > -		cpu_relax();
> 
> Could you elaborate, why you had to remove this?
> 
> Busy waiting loops should always do cpu_relay()
> to reduce heat procution and power consumption while idle.

I removed it because its not idling anymore in that loop imho. It reads
data from MMIO and does a small calculation with it.

Joerg

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