[PATCH] ARM: relax conditions required for enabling Contiguous Memory Allocator

Hello,

On Tuesday, August 21, 2012 2:13 PM Russell King - ARM Linux wrote:

On Mon, Aug 20, 2012 at 08:01:23PM +0000, Arnd Bergmann wrote:

quoted

The patch looks simple, but I want a better explanation for it.
When we went through all possible cases, we decided that:

* ARMv6+ need CMA to avoid the double mapping problem.
* ARMv4/v5 cannot generally use CMA because it doesn't work
  together with DMABOUNCE. I don't remember if it was the
  only problem, but I definitely remember this was intentional.
* We want a common kernel for all ARMv6+ eventually, and a
  separate kernel for all ARMv4/v5 ones.

If the reasoning has changed, please try to explain the full
situation.

Indeed.

quoted

On a related topic, what happened to the idea that
ARMv6+ is broken without CMA? I noticed that it's optional
now.

With Marek's patch, it's always selected for MMU-based builds (it can't
be disabled).  Before the patch, it was always selected for V6 and later
CPUs.

My patch only alters dependences of HAVE_DMA_CONTIGUOUS symbol, by changing
them from CPU_V6+ to MMU. It doesn't change or select CMA for any of the 
systems - this is done by the CONFIG_CMA symbol from drivers/base/KConfig 
which depends on HAVE_DMA_CONTIGUOUS. It is up to user to enable it or not.

And the description doesn't make sense:

"Contiguous Memory Allocator requires only paging and MMU enabled not
particular CPU architectures,"

what does "only paging and MMU enabled" mean?  Are you trying to say that
CMA only requires a kernel with MMU support?

On ARM architecture CMA can be enabled on any system which has MMU support, 
MMU is required for page migration. The integration layer in dma-mapping is 
generic enough to work on any ARM architecture.

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