On Wed, Jul 06, 2011 at 04:51:49PM +0200, Arnd Bergmann wrote:
On Wednesday 06 July 2011, Russell King - ARM Linux wrote:
On Wed, Jul 06, 2011 at 04:09:29PM +0200, Arnd Bergmann wrote:
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.
One of the requirements of the allocator is that the returned memory should be zero'd (because it can be exposed to userspace via ALSA and frame buffers.)
Zeroing the memory from all the contexts which dma_alloc_coherent is called from is a trivial matter if its in lowmem, but highmem is harder.
I don't see how. The pages get allocated from an unmapped area or memory, mapped into the kernel address space as uncached or wc and then cleared. This should be the same for lowmem or highmem pages.
You don't want to clear them via their uncached or WC mapping, but via their cached mapping _before_ they get their alternative mapping, and flush any cached out of that mapping - both L1 and L2 caches.
For lowmem pages, that's easy. For highmem pages, they need to be individually kmap'd to zero them etc. (alloc_pages() warns on GFP_HIGHMEM + GFP_ZERO from atomic contexts - and dma_alloc_coherent must be callable from such contexts.)
That may be easier now that we don't have the explicit indicies for kmap_atomics, but at that time it wasn't easily possible.
Another issue is that when a platform has restricted DMA regions, they typically don't fall into the highmem zone. As the dmabounce code allocates from the DMA coherent allocator to provide it with guaranteed DMA-able memory, that would be rather inconvenient.
True. The dmabounce code would consequently have to allocate the memory through an internal function that avoids the contiguous allocation area and goes straight to ZONE_DMA memory as it does today.
CMA's whole purpose for existing is to provide _dma-able_ contiguous memory for things like cameras and such like found on crippled non- scatter-gather hardware. If that memory is not DMA-able what's the point?