On Fri, Apr 26, 2013 at 10:41:23AM +0100, Russell King - ARM Linux wrote: > On Fri, Apr 26, 2013 at 01:46:46PM +0530, Vinod Koul wrote: > > On Fri, Apr 26, 2013 at 10:28:39AM +0200, Linus Walleij wrote: > > > On Thu, Apr 25, 2013 at 4:11 PM, Arnd Bergmann <a...@arndb.de> wrote: > > > > > > > The dma engine driver must know the address in its dma space, while the > > > > slave driver has it available in physical space. These two are often the > > > > same, but there is no generic way to convert between the two, especially > > > > if the dma engine resides behind an IOMMU. > > > > > > > > The best assumption we can make is that the dma engine driver knows > > > > how to convert between the two. Interestingly the documentation for > > > > dma_slave_config talks about "physical address", while the structure > > > > itself uses a dma_addr_t. Linus Walleij introduced the structure in > > > > c156d0a5b0 "DMAENGINE: generic slave channel control v3", so I assume > > > > he can shed some light on what he was thinking. I assume the > > > > documentation > > > > is right but the structure is not and should be converted to use > > > > phys_add_t or resource_size_t. > > > > > > OK I could cook a patch for that, but I think I need some input from > > > Vinod and/or Russell on this. > > the dma_slave_config is physical address that should be passed directly to > > the > > controller. Obviosuly it should phys_addr_t :) > > What you've just said is actually confusing. > > "physical address" is normally the term used to describe the addresses > seen to the RAM. phys_addr_t describes this. This is not necessarily > what needs to be programmed into the DMA controller. Yes that would be true when you have MMU > > For RAM addresses, they must be mapped via the DMA API - and this gives > you a dma_addr_t. > > "DMA address" is the address to be programmed into a DMA controller to > access a particular address in RAM or device, and has type dma_addr_t. > When you're programming a DMA controller to access a device, you are > clearly telling it the address on the _DMA controller's bus_ to access > that register, which may or may not be the same as the physical address. > > There are platforms in existence where phys_addr_t can be 32-bit but > dma_addr_t can be 64-bit. Getting this stuff wrong can cause problems. Sure, thanks for pointing, so we wont do this change.
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