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? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>