On 10/01/18 15:32, Christoph Hellwig wrote: > 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? Good point - other than the IXP4xx platform and possibly the Broadcom network drivers, it's probably only x86-relevant stuff where the concern is moot. Let's just keep the simple assumption then, until actually proven otherwise. Robin.
