[PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings

[Arnd Bergmann]

On Tuesday 16 August 2011, Russell King - ARM Linux wrote:
> On Tue, Aug 16, 2011 at 03:28:48PM +0200, Arnd Bergmann wrote:
> > Hmm, I don't remember the point about dynamically sizing the pool for
> > ARMv6K, but that can well be an oversight on my part.  I do remember the
> > part about taking that memory pool from the CMA region as you say.
> 
> If you're setting aside a pool of pages, then you have to dynamically
> size it.  I did mention during our discussion about this.
> 
> The problem is that a pool of fixed size is two fold: you need it to be
> sufficiently large that it can satisfy all allocations which come along
> in atomic context.  Yet, we don't want the pool to be too large because
> then it prevents the memory being used for other purposes.
> 
> Basically, the total number of pages in the pool can be a fixed size,
> but as they are depleted through allocation, they need to be
> re-populated from CMA to re-build the reserve for future atomic
> allocations.  If the pool becomes larger via frees, then obviously
> we need to give pages back.

Ok, thanks for the reminder. I must have completely missed this part
of the discussion.

When I briefly considered this problem, my own conclusion was that
the number of atomic DMA allocations would always be very low
because they tend to be short-lived (e.g. incoming network packets),
so we could ignore this problem and just use a smaller reservation
size. While this seems to be true in general (see "git grep -w -A3 
dma_alloc_coherent | grep ATOMIC"), there is one very significant
case that we cannot ignore, which is pci_alloc_consistent.

This function is still called by hundreds of PCI drivers and always
does dma_alloc_coherent(..., GFP_ATOMIC), even for long-lived
allocations and those that are too large to be ignored.

So at least for the case where we have PCI devices, I agree that
we need to have the dynamic pool.

	Arnd