[RFC 4/4] drm: Add NVIDIA Tegra support

[Arnd Bergmann]

On Wednesday 11 April 2012, Thierry Reding wrote:
>   * Daniel Vetter wrote:
> > On Wed, Apr 11, 2012 at 03:23:26PM +0200, Thierry Reding wrote:
> > > * Daniel Vetter wrote:
> > > > On Wed, Apr 11, 2012 at 02:10:30PM +0200, Thierry Reding wrote:
> > > > > This commit adds a very basic DRM driver for NVIDIA Tegra SoCs. It
> > > > > currently has rudimentary GEM support and can run a console on the
> > > > > framebuffer as well as X using the xf86-video-modesetting driver.
> > > > > Only the RGB output is supported. Quite a lot of things still need
> > > > > to be worked out and there is a lot of room for cleanup.
> > > > 
> > > > Indeed, after a quick look there are tons of functions that are just stubs
> > > > ;-) One thing I wonder though is why you directly use the iommu api and
> > > > not wrap it up into dma_map? Is arm infrastructure just not there yet or
> > > > do you plan to tightly integrate the tegra drm with the iommu (e.g. for
> > > > process space switching or similarly funky stuff)?
> > > 
> > > I'm not sure I know what you are referring to. Looking for all users of
> > > iommu_map() doesn't turn up anything related to dma_map. Can you point me in
> > > the right direction?
> > 
> > Well, you use the iommu api to map/unmap memory into the iommu for tegra,
> > whereas usually device drivers just use the dma api to do that. The usual
> > interface is dma_map_sg/dma_unmap_sg, but there are quite a few variants
> > around. I'm just wondering why this you've choosen this.
> 
> I don't think this works on ARM. Maybe I'm not seeing the whole picture but
> judging by a quick look through the kernel tree there aren't any users that
> map DMA memory through an IOMMU.


dma_map_sg is certainly the right interface to use, and Marek Szyprowski has
patches to make that work on ARM, hopefully going into v3.5, so you could
use those.

	Arnd