On Wednesday, January 4, 2017 6:29:39 PM CET Nikita Yushchenko wrote: > > Just a guess, but if the inbound translation windows in the host > > bridge are wider than 32-bit, the reason for setting up a single > > 32-bit window is probably because that is what the parent bus supports. > > Well anyway applying patch similar to your's will fix pcie-rcar + nvme > case - thus I don't object :) But it can break other cases ... > > But why do you hook at set_dma_mask() and overwrite mask inside, instead > of hooking at dma_supported() and rejecting unsupported mask? > > I think later is better, because it lets drivers to handle unsupported > high-dma case, like documented in DMA-API_HOWTO. I think the behavior I put in there is required for swiotlb to make sense, otherwise you would rely on the driver to handle dma_set_mask() failure gracefully with its own bounce buffers (as network and scsi drivers do but others don't). Having swiotlb or iommu enabled should result in dma_set_mask() always succeeding unless the mask is too small to cover the swiotlb bounce buffer area or the iommu virtual address space. This behavior is particularly important in case the bus address space is narrower than 32-bit, as we have to guarantee that the fallback to 32-bit DMA always succeeds. There are also a lot of drivers that try to set a 64-bit mask but don't implement bounce buffers for streaming mappings if that fails, and swiotlb is what we use to make those drivers work. And yes, the API is a horrible mess. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
