linux-kernel - Re: [PATCH -mm] x86 calgary: fix handling of devces that aren't behind the Calgary

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Date:	Tue, 3 Jun 2008 08:21:46 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	linux-kernel@...r.kernel.org, alexisb@...ibm.com, mingo@...e.hu,
	akpm@...ux-foundation.org
Subject: Re: [PATCH -mm] x86 calgary: fix handling of devces that aren't
	behind the Calgary

On Sat, May 31, 2008 at 01:31:33PM +0900, FUJITA Tomonori wrote:

> The calgary code can give drivers addresses above 4GB which is very
> bad for hardware that is only 32bit DMA addressable:
> 
> http://lkml.org/lkml/2008/5/8/423
> 
> This patch tries to fix the problem by using per-device
> dma_mapping_ops support. This fixes the calgary code to use swiotlb
> or nommu properly for devices which are not behind the
> Calgary/CalIOC2.
> 
> With this patch, the calgary code sets the global dma_ops to swiotlb
> or nommu, and the dma_ops of devices behind the Calgary/CalIOC2 to
> calgary_dma_ops. So the calgary code can handle devices safely that
> aren't behind the Calgary/CalIOC2.

This seems a little backward to me. I thought we were going to get rid
of the global dma_ops? If not, assuming going through the global one
would be more efficient, Calgary should be the global one and
nommu/swiotlb should be used on devices that do not have translation
enabled. The reason why is that the majority of devices on a Calgary
system, assuming Calgary is in use, will have translation enabled.

In general the patch looks good, barring the point above. We'll give
it a spin on some Calgary/CalIOC2 machines.

Cheers,
Muli


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