On Wed, Oct 23, 2024 at 08:12:33AM +0200, Christoph Hellwig wrote: > On Tue, Oct 22, 2024 at 11:24:31AM +0100, Catalin Marinas wrote: > > > > We should not allow smaller than cache line alignment on architectures > > > > that are not cache coherent indeed. > > > > Even on architectures that are not fully coherent, the coherency is a > > property of the device. You may need to somehow pass this information in > > struct queue_limits if you want it to be optimal. > > Well, devices set the queue limits. So this would be a fix in the > drivers that set the queue limits. SCSI already does this in the > midlayer code, I guess it isn't true: [linux]# cat /sys/block/sda/queue/dma_alignment 3 > so the main places to fix are nvme und ublk. > > I cant take care of nvme by copying the scsi pattern. > > > That said, the DMA debug code also uses the static L1_CACHE_SHIFT and it > > will trigger the warning anyway. Some discussion around the DMA API > > debug came up during the small ARCH_KMALLOC_MINALIGN changes (don't > > remember it was in private with Robin or on the list). Now kmalloc() can > > return a small buffer (less than a cache line) that won't be bounced if > > the device is coherent (see dma_kmalloc_safe()) but the DMA API debug > > code only checks for direction == DMA_TO_DEVICE, not > > dev_is_dma_coherent(). For arm64 I did not want to disable small > > ARCH_KMALLOC_MINALIGN if CONFIG_DMA_API_DEBUG is enabled as this would > > skew the testing by forcing all allocations to be ARCH_DMA_MINALIGN > > aligned. > > > > Maybe I'm missing something in those checks but I'm surprised that the > > DMA API debug code doesn't complain about small kmalloc() buffers on x86 > > (which never had any bouncing for this specific case since it's fully > > coherent). I suspect people just don't enable DMA debugging on x86 for > > such devices (typically USB drivers have this issue). > > I don't think there's too many of these indeed. Usually it is assumed that it is safe to DMA over kmalloc() buffer... thanks, Ming
