Messages in this thread | | | Date | Thu, 28 Apr 2011 11:51:31 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1 |
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On Thu, Apr 28, 2011 at 12:32:32PM +0200, Marek Szyprowski wrote: > On Thursday, April 28, 2011 11:38 AM Russell King - ARM Linux wrote: > > > > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out > > > > > that this is needed, and it currently is not implemented, with > > > > > an outdated comment explaining why it used to not be possible > > > > > to do it. > > > > > > > > dma_alloc_noncoherent is an entirely pointless API afaics. > > > > > > I was about to ask what the point is ... (what is the expected > > > semantic ? Memory that is reachable but not necessarily cache > > > coherent ?) > > > > As far as I can see, dma_alloc_noncoherent() should just be a wrapper > > around the normal page allocation function. I don't see it ever needing > > to do anything special - and the advantage of just being the normal > > page allocation function is that its properties are well known and > > architecture independent. > > If there is IOMMU chip that supports pages larger than 4KiB then > dma_alloc_noncoherent() might try to allocate such larger pages what will > result in faster access to the buffer (lower iommu tlb miss ratio). > For large buffers even 64KiB 'pages' gives a significant performance > improvement.
The memory allocated by dma_alloc_noncoherent() (and dma_alloc_coherent()) has to be virtually contiguous, and DMA contiguous. It is assumed by all drivers that:
virt = dma_alloc_foo(size, &dma);
cpuaddr = virt + offset; dmaaddr = dma + offset;
results in the CPU and DMA seeing ultimately the same address for cpuaddr and dmaaddr for 0 <= offset < size.
The standard alloc_pages() also ensures that if you ask for an order-N page, you'll end up with that allocation being contiguous - so there's no difference there.
What I'd suggest is that dma_alloc_noncoherent() should be architecture independent, and should call into whatever iommu support the device has to setup an approprite iommu mapping. IOW, I don't see any need for every architecture to provide its own dma_alloc_noncoherent() allocation function - or indeed every iommu implementation to deal with the allocation issues either.
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