On Wed, Jan 10, 2018 at 11:49:34AM +0000, Robin Murphy wrote:

quoted

+#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?