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

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Message-ID: <20140508001814.GB4951@google.com>
Date:	Wed, 7 May 2014 18:18:14 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Arnd Bergmann <arnd@...db.de>
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 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?

Bjorn
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