Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments

[Bjorn Helgaas <bhelgaas@google.com>]

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