Re: [PATCH 6/8] drivers: add Contiguous Memory Allocator

[Russell King - ARM Linux]

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?
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