On 3/18/2016 10:20 AM, Boris Brezillon wrote: > 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)? > > Sure, that's fine for me. -- Sinan Kaya Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project -- 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
