Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 03 February 2015 09:04:03 Rob Clark wrote:
> On Tue, Feb 3, 2015 at 2:48 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote:
> >> On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> >> >> My initial thought is for dma-buf to not try to prevent something than
> >> >> an exporter can actually do.. I think the scenario you describe could
> >> >> be handled by two sg-lists, if the exporter was clever enough.
> >> >
> >> > That's already needed, each attachment has it's own sg-list. After all
> >> > there's no array of dma_addr_t in the sg tables, so you can't use one sg
> >> > for more than one mapping. And due to different iommu different devices
> >> > can easily end up with different addresses.
> >>
> >>
> >> Well, to be fair it may not be explicitly stated, but currently one
> >> should assume the dma_addr_t's in the dmabuf sglist are bogus.  With
> >> gpu's that implement per-process/context page tables, I'm not really
> >> sure that there is a sane way to actually do anything else..
> >
> > Hm, what does per-process/context page tables have to do here? At least on
> > i915 we have a two levels of page tables:
> > - first level for vm/device isolation, used through dma api
> > - 2nd level for per-gpu-context isolation and context switching, handled
> >   internally.
> >
> > Since atm the dma api doesn't have any context of contexts or different
> > pagetables, I don't see who you could use that at all.
> 
> Since I'm stuck w/ an iommu, instead of built in mmu, my plan was to
> drop use of dma-mapping entirely (incl the current call to dma_map_sg,
> which I just need until we can use drm_cflush on arm), and
> attach/detach iommu domains directly to implement context switches.
> At that point, dma_addr_t really has no sensible meaning for me.

I think what you see here is a quite common hardware setup and we really
lack the right abstraction for it at the moment. Everybody seems to
work around it with a mix of the dma-mapping API and the iommu API.
These are doing different things, and even though the dma-mapping API
can be implemented on top of the iommu API, they are not really compatible.

The drm_clflush helpers don't seem like the right solution to me,
because all other devices outside of drm will face the same issue,
and I suspect we should fill the missing gaps in the API in a
more generic way.

	Arnd