Hello, On Wednesday, August 17, 2011 10:01 AM Marek Szyprowski wrote: > On Tuesday, August 16, 2011 4:26 PM Arnd Bergmann wrote: > > 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. > > Do we really need the dynamic pool for the first version? I would like to > know how much memory can be allocated in GFP_ATOMIC context. What are the > typical sizes of such allocations? > > Maybe for the first version a static pool with reasonably small size > (like 128KiB) will be more than enough? This size can be even board > depended or changed with kernel command line for systems that really > needs more memory. > > I noticed one more problem. The size of the CMA managed area must be > the multiple of 16MiBs (MAX_ORDER+1). This means that the smallest CMA area > is 16MiB. These values comes from the internals of the kernel memory > management design and page blocks are the only entities that can be managed > with page migration code. I'm really sorry for the confusion. This 16MiB value worried me too much and I've checked the code once again and found that this MAX_ORDER+1 value was a miscalculation, which appeared in v11 of the patches. The true minimal CMA area size is 8MiB for ARM architecture. I believe this shouldn't be an issue for the current ARMv6+ based machines. I've checked it with "mem=16M cma=8M" kernel arguments. System booted fine and CMA area has been successfully created. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- 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
