linux-kernel - Re: [PATCH v2 4/5] iommu: Use dma_addr

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Message-ID: <15249748.Q2CaU9aBE9@wuerfel>
Date:	Thu, 08 May 2014 12:44:12 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	linux-doc@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joerg Roedel <joro@...tes.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	linux-kernel@...r.kernel.org,
	James Bottomley <jbottomley@...allels.com>,
	linux-pci@...r.kernel.org, David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments

On Wednesday 07 May 2014 18:18:14 Bjorn Helgaas wrote:
> On Wed, May 07, 2014 at 09:58:58AM +0200, Arnd Bergmann wrote:
> > On Tuesday 06 May 2014 16:48:40 Bjorn Helgaas wrote:
> > > Convert the "iova" arguments of iommu_map(), iommu_unmap(), etc., from
> > > "unsigned long" to dma_addr_t.
> > > 
> > > bb5547acfcd8 ("iommu/fsl: Make iova dma_addr_t in the iommu_iova_to_phys
> > > API") did this for iommu_iova_to_phys(), but didn't fix the rest of the
> > > IOMMU API.
> > 
> > This patch looks 100% correct, but I'm not convinced it's a good idea:
> > On 32-bit platforms (i.e. most of the ones you change), doing 64-bit
> > arithmetic has a noticeable overhead. I am not aware of an IOMMU that
> > actually uses 64-bit DMA addresses on its slave side, usually they
> > are used to translate addresses from 32-bit masters into 64-bit
> > memory addresses, so using 'unsigned long' seems better from a practical
> > point of view as opposed to the strict type correctness.
> 
> The current x86 IOMMUs support DMA addresses larger than 32 bits, but
> obviously those platforms usually run 64-bit kernels so "unsigned
> long" is already 64 bits.
> 
> I guess you're thinking about cases where "unsigned long" is 32 bits,
> the IOMMU only supports 32 bit DMA addresses, and dma_addr_t is 64
> bits.  If the IOMMU only supports 32-bit DMA addresses, is there any
> value in having a 64-bit dma_addr_t?

Two cases:

a) You can have a system with some DMA masters that are 64-bit capable,
   and other masters that can only do 32-bit DMA and for this reason
   use an IOMMU. I expect to see more of these in the future.

b) We build kernels that run on a multitude of ARM32 platforms these
   days. We have some that require a 32-bit dma_addr_t and some that
   don't (because they always use swiotlb or an IOMMU).

	Arnd
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