Messages in this thread | | | Date | Wed, 7 May 2014 18:18:14 -0600 | From | Bjorn Helgaas <> | Subject | Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments |
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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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