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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
