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

[Lucas Stach <dev-8ppwABl0HbeELgA04lAiVw@public.gmane.org>]

Am Mittwoch, den 11.04.2012, 15:18 +0000 schrieb 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.

Just jumping in here to make sure everyone understands the limitations
of the Tegra 2 GART IOMMU we are talking about here. It has no isolation
capabilities and a really small remapping window of 32MB. So it's
impossible to remap every buffer used by the graphics engines. The only
sane way to handle this is to set aside a chunk of stolen system memory
as VRAM and let a memory manager like TTM handle the allocation of
linear regions and GART mappings. This means a more tight integration of
the DRM driver and the IOMMU, where I think that using the IOMMU API
directly and completely controlling the GART from one driver is the
right way to go for a number of reasons, where my biggest concern is
that we can't implement a sane out-of-remapping space when we go through
the dma_map API.

It's too late for me to go into the details now, but I wanted to make it
clear that I think that using the IOMMU only and exclusively from the
DRM driver with a high level of tie in is the way to go. If you want to
know more details I'm available to discuss this matter in the next days.

-- Lucas
> 
> 	Arnd
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