On 06/15/2011 12:37 AM, Arnd Bergmann wrote:

On Wednesday 15 June 2011 09:11:39 Marek Szyprowski wrote:

quoted

I see your concerns, but I really wonder how to determine the properties
of the global/default cma pool. You definitely don't want to give all
available memory o CMA, because it will have negative impact on kernel
operation (kernel really needs to allocate unmovable pages from time to
time).

Exactly. This is a hard problem, so I would prefer to see a solution for
coming up with reasonable defaults.

Is this a situation where passing the information from device tree might 
help? I know this does not help short term, but I am trying to 
understand the sorts of problems device tree can help solve.

Philip

quoted

The only solution I see now is to provide Kconfig entry to determine
the size of the global CMA pool, but this still have some issues,
especially for multi-board kernels (each board probably will have
different amount of RAM and different memory-consuming devices
available). It looks that each board startup code still might need to
tweak the size of CMA pool. I can add a kernel command line option for
it, but such solution also will not solve all the cases (afair there
was a discussion about kernel command line parameters for memory
configuration and the conclusion was that it should be avoided).

The command line option can be a last resort if the heuristics fail,
but it's not much better than a fixed Kconfig setting.

How about a Kconfig option that defines the percentage of memory
to set aside for contiguous allocations?

	Arnd

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