On Mon, Apr 12, 2010 at 10:59:57AM +0200, Andi Kleen wrote: > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes: > >> > >> The fix is to use usb_buffer_alloc() for that purpose which ensures > >> memory that is suitable for DMA. And on x86_64, this also means that the > >> upper 32 bits of the address returned are all 0's. > > > > That is not a good fix. usb_buffer_alloc() provides coherent memory, > > which is not what we want. I believe the correct fix is to specify the > > GFP_DMA32 flag in the kzalloc() call. > > The traditional way to handle this is to leave it to swiotlb in > pci_map_*. pci_map_* is needed anyways if you run with a IOMMU. > > Also note at least on x86 systems coherent memory is the same as non coherent > memory. And GFP_DMA32 is a x86 specific flag, doesn't necessarily > do any good anywhere else. > > So if you add x86isms anyways you could as well use dma_alloc_coherent() > directly which is actually better at this than a simple GFP_DMA32 > and as a bonus handles the IOMMUs correctly too. Which is exactly what usb_buffer_alloc() does already. So at least for x86 you say this is the right thing to do? However, we don't necessarily need coherent memory on other platforms, which is why I hessitate to enforce that type of memory for all transfer_buffer allocations. > Or just use GFP_KERNEL and pci_map_* later. The USB core does this already, but at least on Pedro's machine, this seems unsufficient. Unfortunately, I can't reproduce the issue yet, even with more than 4GB of RAM installed. Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
