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.
