On Thu, 2019-07-18 at 13:18 +0200, Nicolas Saenz Julienne wrote: > On Thu, 2019-07-18 at 11:15 +0200, Christoph Hellwig wrote: > > On Wed, Jul 17, 2019 at 05:31:34PM +0200, Nicolas Saenz Julienne wrote: > > > Historically devices with ZONE_DMA32 have been assumed to be able to > > > address at least the lower 4GB of ram for DMA. This is still the defualt > > > behavior yet the Raspberry Pi 4 is limited to the first GB of memory. > > > This has been observed to trigger failures in dma_direct_supported() as > > > the 'min_mask' isn't properly set. > > > > > > We create 'dma_direct_min_mask' in order for the arch init code to be > > > able to fine-tune dma direct's 'min_dma' mask. > > > > Normally we use ZONE_DMA for that case. > > Fair enough, I didn't think of that possibility. > > So would the arm64 maintainers be happy with something like this: > > - ZONE_DMA: Follows standard definition, 16MB in size. ARCH_ZONE_DMA_BITS is > left as is. > - ZONE_DMA32: Will honor the most constraining 'dma-ranges'. Which so far for > most devices is 4G, except for RPi4. > - ZONE_NORMAL: The rest of the memory. Never mind this suggestion, I don't think it makes any sense. If anything arm64 seems to fit the ZONE_DMA usage pattern of arm and powerpc: where ZONE_DMA's size is decided based on ram size and/or board configuration. It was actually set-up like this until Christoph's ad67f5a6545f7 ("arm64: replace ZONE_DMA with ZONE_DMA32"). So the easy solution would be to simply revert that commit. On one hand I feel it would be a step backwards as most 64 bit architectures have been moving to use ZONE_DMA32. On the other, current ZONE_DMA32 usage seems to be heavily rooted on having a 32 bit DMA mask*, which will no longer be the case on arm64 if we want to support the RPi 4. So the way I see it and lacking a better solution, the argument is stronger on moving back arm64 to using ZONE_DMA. Any comments/opinions? Note that I've been looking at all the DMA/CMA/swiotlb code to see if this would break anything or change behaviors and couldn't find anything obvious. I also tested the revert on my RPi4 and nothing seems to fail. * A good example is dma-direct's implementation.