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

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Message-Id: <200808142035.32422.ioe-lkml@rameria.de>
Date:	Thu, 14 Aug 2008 20:35:30 +0200
From:	Ingo Oeser <ioe-lkml@...eria.de>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	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

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.

> +		/* wait for the bit to become one */
> +		status = readl(iommu->mmio_base + MMIO_STATUS_OFFSET);
> +		ready = status & MMIO_STATUS_COM_WAIT_INT_MASK;
>  	}
>  

Best Regards

Ingo Oeser
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