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    SubjectRe: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with dma_map_single
    On Fri, 18 Mar 2016 11:25:48 +0000
    Robin Murphy <robin.murphy@arm.com> wrote:

    > On 18/03/16 09:30, Boris Brezillon wrote:
    > > On Thu, 17 Mar 2016 23:50:20 +0000
    > > Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:
    > >
    > >> On Thu, Mar 17, 2016 at 07:17:24PM -0400, okaya@codeaurora.org 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
    >

    Hm, interesting, thanks for the pointer.

    --
    Boris Brezillon, Free Electrons
    Embedded Linux and Kernel engineering
    http://free-electrons.com

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