[Linaro-mm-sig] Minutes from V4L2 update call

[Daniel Vetter]

On Thu, Mar 22, 2012 at 11:54:55AM -0300, Christian Robottom Reis wrote:
>     Tomasz: proposed extension to DMA Mapping -- dma_get_pages
>         Currently difficult to change the camera address into list of pages
>         DMA framework has the knowledge of this list and could do this
> 
>             Depends on dma_get_pages
>             Needs to be merged first
> 
>         Test application posted to dri-devel with dependencies to run demo
>             Many dependencies

I kinda missed to yell at this patch when it first showed up, so I'll do
that here ;-)

I think this is a gross layering violation and I don't like it at all. The
entire point of the dma api is that device drivers only get to see device
addresses and can forget about all the remapping/contig-alloc madness. And
dma-buf should just follow this with it's map/unmap interfaces.

Furthermore the exporter memory might simply not have any associated
struct pages. The two examples I always bring up:
- special purpose remapping units (like omap's TILER) which are managed by
  the exporter and can do crazy things like tiling or rotation
  transparently for all devices.
- special carve-out memory which is unknown to linux memory management.
  drm/i915 is totally abusing this, mostly because windows is lame and
  doesn't have decent largepage allocation support. This is just plain
  system memory, but there's no struct page for it (because it's not part
  of the system map).

Now the core dma api isn't fully up to snuff for everything yet and there
are things missing. But it's certainly not dma_get_pages, but more things
like mmap support for coherent memory or allocating coherent memroy which
doesn't have a static mapping in the kernel address space. I very much
hope that the interfaces we develop for dma-buf (and the insights gained)
could help as examples here, so that in the further there's not such a
gaping difference for the driver between dma_coherent allocations of it's
own and imported buffer objects.

Yours, Daniel
-- 
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48