On Tue, Jan 10, 2017 at 01:25:12PM +0000, Robin Murphy wrote: > We still need a way for drivers to communicate a device's probed > addressing capability to SWIOTLB, so there's always going to have to be > *some* sort of public interface. Personally, the change in semantics I'd > like to see is to make dma_set_mask() only fail if DMA is entirely > disallowed - in the normal case it would always succeed, but the DMA API > implementation would be permitted to set a smaller mask than requested > (this is effectively what the x86 IOMMU ops do already). Yes, this sounds reasonable. > The significant > work that would require, though, is changing all the drivers currently > using this sort of pattern: > > if (!dma_set_mask(dev, DMA_BIT_MASK(64)) > /* put device into 64-bit mode */ > else if (!dma_set_mask(dev, DMA_BIT_MASK(32)) > /* put device into 32-bit mode */ > else > /* error */ While we have this pattern in a lot of places it's already rather pointless on most architectures as the first dma_set_mask call won't ever fail for the most common dma_ops implementations. > to something like this: > > if (!dma_set_mask(dev, DMA_BIT_MASK(64)) > /* error */ > if (dma_get_mask(dev) > DMA_BIT_MASK(32)) > /* put device into 64-bit mode */ > else > /* put device into 32-bit mode */ > > Which would be a pretty major job. I don't think it's too bad. Also for many modern devices there is no need to put the device into a specific mode. It's mostly a historic issue from the PCI/PCI-X days with the less efficient DAC addressing scheme.
