Tegra DRM device tree bindings

[Lucas Stach]

Am Mittwoch, den 27.06.2012, 16:44 +0200 schrieb Thierry Reding:
> On Wed, Jun 27, 2012 at 05:29:14PM +0300, Hiroshi Doyu wrote:
> > On Wed, 27 Jun 2012 16:08:10 +0200
> > Thierry Reding <thierry.reding at avionic-design.de> wrote:
> > 
> > > * PGP Signed by an unknown key
> > > 
> > > On Wed, Jun 27, 2012 at 03:59:07PM +0300, Hiroshi Doyu wrote:
> > > > On Wed, 27 Jun 2012 07:14:18 +0200
> > > > Thierry Reding <thierry.reding at avionic-design.de> wrote:
> > > > 
> > > > > > Old 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 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?
> > > > 
> > > > 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.
> > > 
> > > I was hoping that we could get away with using the CMA and perhaps
> > > initialize it based on device tree content. I agree that the carveout
> > > size depends on the use-case, but I still think it makes sense to
> > > specify it on a per-board basis.
> > 
> > DRM driver doesn't know if it uses CMA or not, because DRM only uses
> > DMA API.
> 
> So how is the DRM supposed to allocate buffers? Does it call the
> dma_alloc_from_contiguous() function to do that? I can see how it is
> used by arm_dma_ops but how does it end up in the driver?
> 
As I said before the DMA API is not a good fit for graphics drivers.
Most of the DMA buffers used by graphics cores are long lived and big,
so we need a special pool to alloc from to avoid eating all contiguous
address space, as DMA API does not provide shrinker callbacks for
clients using large amount of memory.

> > I think that "coherent_pool" can be used only when the amount of
> > contiguous memory is short in your system. Otherwise even unnecessary.
> > 
> > Could you explain a bit more why you want carveout size on per-board basis?
> 
> In the ideal case I would want to not have a carveout size at all.
> However there may be situations where you need to make sure some driver
> can allocate a given amount of memory. Having to specify this using a
> kernel command-line parameter is cumbersome because it may require
> changes to the bootloader or whatever. So if you know that a particular
> board always needs 128 MiB of carveout, then it makes sense to specify
> it on a per-board basis.

If we go with CMA, this is a non-issue, as CMA allows to use the contig
area for normal allocations and only purges them if it really needs the
space for contig allocs.
> 
> Thierry