linux-kernel - Re: [PATCH 04/13 v2] iommu/arm-smmu: Convert to iommu

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Date:	Mon, 22 Sep 2014 17:36:28 +0200
From:	Joerg Roedel <jroedel@...e.de>
To:	Will Deacon <will.deacon@....com>
Cc:	Joerg Roedel <joro@...tes.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 04/13 v2] iommu/arm-smmu: Convert to iommu_capable() API
 function function

Hi Will,

On Fri, Sep 19, 2014 at 05:42:20PM +0100, Will Deacon wrote:
> However, I thought about this a bit more and the coherency isn't necessarily
> a global property of the SMMU. In reality, it is dependent on the IOTLBs in
> use by the domain, so it's not going to be possible to report true here in
> many cases.
> 
> That means we'd need a way to say `this device is dma coherent when its
> downstream IOMMU is enabled with IOMMU_CACHE mappings'. For the moment,
> people will probably just add `dma-coherent' to the endpoint and dma-mapping
> will request IOMMU_CACHE mappings regardless of the features advertised by
> the IOMMU. In that case, it might make more sense to return `true' here as
> we can always generated cacheable transactions from the SMMU. The
> dma-coherent property on the device would then indicate whether those
> transactions will snoop the CPU caches.

Okay, when the SMMU can always generate cachable transactions it is
surely fine to return true here. I'll change that before I add the
patches to my tree.


	Joerg

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