On Fri, 18 Mar 2016 09:51:37 -0400 Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote: > On 3/18/2016 7:25 AM, Robin Murphy wrote: > > On 18/03/16 09:30, Boris Brezillon wrote: > >> On Thu, 17 Mar 2016 23:50:20 +0000 > >> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > >> > >>> On Thu, Mar 17, 2016 at 07:17:24PM -0400, okaya@xxxxxxxxxxxxxx wrote: > >>>> What is the correct way? I don't want to write engine->sram_dma = sram > >>> > >>> Well, what the driver _is_ wanting to do is to go from a CPU physical > >>> address to a device DMA address. phys_to_dma() looks like the correct > >>> thing there to me, but I guess that's just an offset and doesn't take > >>> account of any IOMMU that may be in the way. > >>> > >>> If you have an IOMMU, then the whole phys_to_dma() thing is a total > >>> failure as it only does a linear translation, and there are no > >>> interfaces in the kernel to take account of an IOMMU in the way. So, > >>> it needs something designed for the job, implemented and discussed by > >>> the normal methods of proposing a new cross-arch interface for drivers > >>> to use. > >>> > >>> What I'm certain of, though, is that the change proposed in this patch > >>> will break current users of this driver: virt_to_page() on an address > >>> returned by ioremap() is completely undefined, and will result in > >>> either a kernel oops, or if not poking at memory which isn't a struct > >>> page, ultimately resulting in something that isn't SRAM being pointed > >>> to by "engine->sram_dma". > >>> > >> > >> Or we could just do > >> > >> engine->sram_dma = res->start; > >> > >> which is pretty much what the SRAM/genalloc code is doing already. > > > > As Russell points out this is yet another type of "set up a DMA master to access something other than kernel RAM" - there's already discussion in progress over how to handle this for dmaengine slaves[1], so gathering more use-cases might help distil exactly what the design of not-strictly-DMA-but-so-closely-coupled-it-can't-really-live-anywhere-else needs to be. > > > > Robin. > > > > [1]:http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/414422.html > > > > Thanks for the link. > > dma_map_resource looks like to be the correct way of doing things. Just from > the purist point of view, a driver is not supposed to know the physical address > of a DMA address. That kills the intent of using DMA API. When programming descriptors, > the DMA addresses should be programmed not physical addresses so that the same > driver can be used in a system with IOMMU. The IOMMU DMA ops will remap the DMA > address to a bus address that is not physical address. All of this operation needs > to be isolated from the device driver. > > > I don't know the architecture or the driver enough to write this. This is not ideally > right but I can do this if Boris you are OK with this. > > engine->sram_dma = res->start; I don't know. How about waiting for the 'dma_{map,unmap}_resource' discussion to settle down before removing phy_to_dma()/dma_to_phys() APIs (as suggested by Robin and Russell)? -- Boris Brezillon, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
