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:
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.
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