Tegra DRM device tree bindings

[Thierry Reding]

On Wed, Jun 27, 2012 at 10:13:56AM +0200, Lucas Stach wrote:
> Hi all,
> 
> I'm not sure what your exact plans are for the direction in which the
> DRM driver should head, as I'm still a bit out of the loop as many of
> those matters were only discussed internally at NVIDIA or with some NDA
> developers. But I'll still try to get into the discussion.
> 
> Am Mittwoch, den 27.06.2012, 07:14 +0200 schrieb Thierry Reding:
> > On Tue, Jun 26, 2012 at 08:48:18PM -0600, Stephen Warren wrote:
> > > On 06/26/2012 08:32 PM, Mark Zhang wrote:
> > > >> On 06/26/2012 07:46 PM, Mark Zhang wrote:
> > > >>>>> On Tue, 26 Jun 2012 12:55:13 +0200
> > > >>>>> Thierry Reding <thierry.reding at avionic-design.de> wrote:
> > > >> ...
> > > >>>> I'm not sure I understand how information about the carveout would be
> > > >>>> obtained from the IOMMU API, though.
> > > >>>
> > > >>> I think that can be similar with current gart implementation. Define carveout as:
> > > >>>
> > > >>> carveout {
> > > >>>         compatible = "nvidia,tegra20-carveout";
> > > >>>         size = <0x10000000>;
> > > >>> };
> > > >>>
> > > >>> Then create a file such like "tegra-carveout.c" to get these definitions and
> > > >> register itself as platform device's iommu instance.
> > > >>
> > > >> The carveout isn't a HW object, so it doesn't seem appropriate to define a DT
> > > >> node to represent it.
> > > > 
> > > > Yes. But I think it's better to export the size of carveout as a configurable item.
> > > > So we need to define this somewhere. How about define carveout as a property of gart?
> > > 
> > > There already exists a way of preventing Linux from using certain chunks
> > > of memory; the /memreserve/ syntax. From a brief look at the dtc source,
> > > it looks like /memreserve/ entries can have labels, which implies that a
> > > property in the GART node could refer to the /memreserve/ entry by
> > > phandle in order to know what memory regions to use.
> > 
> > Wasn't the whole point of using a carveout supposed to be a replacement
> > for the GART? As such I'd think the carveout should rather be a property
> > of the host1x device.
> > 
> In my understanding carveout is neither a hardware nor software
> component. It's just a somewhat special pool of memory. As I pointed out
> in one of the older mails, carveout can not completely replace GART.
> While normal allocations for graphics use should be done contiguous,
> GART allows us to link normal scattered sysram buffers into GPU address
> space, which is a nice thing to have.

Do you agree that this will likely not be a problem with a more or less
stupid framebuffer DRM driver? I recall you mention that in a 3D context
where the scattered buffers are for example geometry provided by OpenGL,
right?

I think we need to put some thoughts into that once we start to
implement more advanced features. At that point we may also want to
think about how to integrate that with TTM.

> IMHO if carveout is to be used exclusively by the GPU (i.e. the DRM
> driver) it should be a property of the host1x device.
> 
> > AIUI what we want to do is have a large contiguous region of memory that
> > a central component (host1x) manages as a pool from which clients (DRM,
> > V4L, ...) can allocate buffers as needed. Since all of this memory will
> > be contiguous anyway there isn't much use for the GART anymore.
> > 
> I think this is the wrong way to go. Having a special memory pool
> managed by some driver adds one more allocator to the kernel, which is
> clearly not desirable. If we want a special mem region for GPU use, we
> should not share this memory pool with other components.
> 
> But if we want a mem region for contig allocations used by many
> components, which seems to be consensus here, CMA is the way to go. In
> this case I think we don't want to bother with the carveout property at
> all at the DRM driver level. Such a shared mem region managed by CMA
> should be defined at a higher level of the device tree.

Okay, this pretty much matches what we've agreed on in another
subthread. CMA looks like the best option for now as it should
cover everything we need at present.

Thierry
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