Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic

[Christoph Hellwig]

On Thu, Oct 19, 2017 at 06:47:45PM -0400, Jim Quinlan wrote:
> The only way to prevent this is to reserve a single page at the end of
> the first memory region of any pair that are adjacent in physical
> memory.  A hack, yes, but I don't see an easier way out of this.  Many
> if not most of our boards do not have adjacent regions and would not
> need this.

dma mappings can be much larger than a single page.  For the block
world take a look at __blk_segment_map_sg which does the merging
of contiguous pages into a single SG segment.  You'd have to override
BIOVEC_PHYS_MERGEABLE to prevent this from happening in your supported
architectures for the block layer.