On Fri, 18 Mar 2016 11:25:48 +0000
Robin Murphy [off-list ref] wrote:

On 18/03/16 09:30, Boris Brezillon wrote:

quoted

On Thu, 17 Mar 2016 23:50:20 +0000
Russell King - ARM Linux [off-list ref] wrote:

quoted

On Thu, Mar 17, 2016 at 07:17:24PM -0400, okaya at codeaurora.org wrote:

quoted

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