Re: [PATCH 1/3] crypto: marvell/cesa - replace dma_to_phys with dma_map_single

[Robin Murphy]

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