Re: Tegra DRM device tree bindings

[Thierry Reding <thierry.reding-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org>]

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On Wed, Jun 27, 2012 at 12:02:46PM -0600, Stephen Warren wrote:
> On 06/27/2012 06:44 AM, Hiroshi Doyu wrote:
> ...
> > I think that there are 2 cases:
> > 
> >   (1) discontiguous memory with IOMMU
> >   (2) contiguous memory without IOMMU(called "carveout" in general?)
> ...
> > For (2), although memory is mostly anonymous one, we may need to know
> > how much to allocate, where we only need "size". This size is not from
> > h/w feature, but it depends on the system load/usage. So I think that
> > this size can be passed from kernel command line? For exmaple, we can
> > specify how much contiguous memory is necessary with putting
> > "coherent_pool=??M" in the kernel command line as below:
> > 
> > 	coherent_pool=nn[KMG]	[ARM,KNL]
> > 			Sets the size of memory pool for coherent, atomic dma
> > 			allocations.
> 
> I guess if that's the standard way of initializing CMA, then that's fine.
> 
> It'd be nice if there was a way to specify that from the DT too; that
> way the user/distro/bootloader constructing the kernel command-line
> wouldn't have to remember to add "random" (potentially
> Tegra-/board-specific) extra arguments onto the command-line; the Tegra
> command-line in the upstream kernel is quite clean right now, especially
> compare to the enormous number of options we require downstream:-(

Looking at Documentation/kernel-parameters.txt it seems the canonical
way to initialize CMA is using the "cma" kernel command-line parameter.
For device tree we could extend the "chosen" node to include something
like "carveout = <0x04000000>;". Or "contiguous-memory" or whatever.

Thierry

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