Re: Tegra DRM device tree bindings

[Hiroshi Doyu <hdoyu-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On Wed, 27 Jun 2012 07:14:18 +0200
Thierry Reding <thierry.reding-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org> wrote:

> * PGP Signed by an unknown key
> 
> 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-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org> 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?

Mostly agree. IIUC, we use both carveout/gart allocated buffers in
android/tegra2.

>As such I'd think the carveout should rather be a property
> of the host1x device.

Rather than introducing a new property, how about using
"coherent_pool=??M" in the kernel command line if necessary? I think
that this carveout size depends on the system usage/load.

> 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.

Right.

I'd think that the amount of contiguous resion might depend on the
system usage/load.