Hello,

On Wednesday, July 06, 2011 4:09 PM Arnd Bergmann wrote:

On Wednesday 06 July 2011, Marek Szyprowski wrote:

quoted

The only problem that might need to be resolved is GFP_ATOMIC allocation
(updating page properties probably requires some locking), but it can be
served from a special area which is created on boot without low-memory
mapping at all. None sane driver will call dma_alloc_coherent(GFP_ATOMIC)
for large buffers anyway.

Would it be easier to start with a version that only allocated from memory
without a low-memory mapping at first?

This would be similar to the approach that Russell's fix for the regular
dma_alloc_coherent has taken, except that you need to also allow the memory
to be used as highmem user pages.

Maybe you can simply adapt the default location of the contiguous memory
are like this:
- make CONFIG_CMA depend on CONFIG_HIGHMEM on ARM, at compile time
- if ZONE_HIGHMEM exist during boot, put the CMA area in there
- otherwise, put the CMA area at the top end of lowmem, and change
  the zone sizes so ZONE_HIGHMEM stretches over all of the CMA memory.

This will not solve our problems. We need CMA also to create at least one
device private area that for sure will be in low memory (video codec).

I will rewrite ARM dma-mapping & CMA integration patch basing on the latest 
ARM for-next patches and add proof-of-concept of the solution presented in my
previous mail (2-level page tables and unmapping pages from low-mem).

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center