On Wed, Jan 10, 2018 at 11:49:34AM +0000, Robin Murphy wrote: >> +#ifdef CONFIG_ZONE_DMA >> + if (mask < DMA_BIT_MASK(ARCH_ZONE_DMA_BITS)) >> + return 0; >> +#else >> + /* >> + * Because 32-bit DMA masks are so common we expect every architecture >> + * to be able to satisfy them - either by not supporting more physical >> + * memory, or by providing a ZONE_DMA32. If neither is the case, the >> + * architecture needs to use an IOMMU instead of the direct mapping. >> + */ >> + if (mask < DMA_BIT_MASK(32)) >> + return 0; > > Do you think it's worth the effort to be a little more accommodating here? > i.e.: > > return dma_max_pfn(dev) >= max_pfn; > > We seem to have a fair few 28-31 bit masks for older hardware which > probably associates with host systems packing equivalently small amounts of > RAM. And those devices don't have a ZONE_DMA? I think we could do something like that, but I'd rather have it as a separate commit with a good explanation. Maybe you can just send on on top of the series?
