Re: [RFC 0/4] dmaengine: rcar-dmac: add iommu support for slave transfers

["Niklas Söderlund" <niklas.soderlund@ragnatech.se>]

Hi Geert,

* Geert Uytterhoeven <geert@linux-m68k.org> [2016-01-08 10:28:22 +0100]:

> Hi Niklas,
>
> On Fri, Jan 8, 2016 at 9:23 AM, Niklas Söderlund
> <niklas.soderlund@ragnatech.se> wrote:
> > What I'm most concerned about is that I could not find a good way too
> > keep track of mappings from different DMA channels to the same address
> > so that they could properly be unmapped. This issue is due to that two
> > channels might both try to map the same address (think rx/tx pair) and
> > that would make it unsafe for any one of the channels to unmap the
> > address without knowing if it's in use by the other.
> >
> > A follow up problem to this is that if the same address is mapped from
> > two different channels but with different sizes and the larger size is
> > tried last. If such a condition should happen and the larger size is so
> > large that it crosses a page boundary there would be trouble.
> >
> > An obvious solution to both problem would be to map address to different
> > iova:s in each channel and keep track of the unique mapping. That way
> > all the above problems would go away. The problem is I can't figure out
> > how to allocate a unique iova to feed to iommu_map.
>
> What about
>   1. Reference counting the mappings,
>   2. Returning -EBUSY if the same address is already mapped with a
>      different size? In the case of RX/TX pairs, the size should be the same
>      (one page).

Thanks for the pointers Geert. I did a implementation using reference
counting, it worked but got quiet messy, it was however not all a waste.
I learned that the dma-mapper api is way smarter then me and provided a
interface that solves multiple mappings of the same address.

// Niklas