On Wed, Jun 27, 2012 at 12:02:46PM -0600, Stephen Warren wrote:

On 06/27/2012 06:44 AM, Hiroshi Doyu wrote:
...

quoted

I think that there are 2 cases:

  (1) discontiguous memory with IOMMU
  (2) contiguous memory without IOMMU(called "carveout" in general?)

...

quoted

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