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

[Jim Quinlan]

On Fri, Oct 20, 2017 at 3:37 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> 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.

I am not sure I understand your comment -- the size of the request
shouldn't be a factor.  Let's look at your example of the DMA request
of 3fffff00 to 4000000f (physical memory).  Lets say it is for 15
pages.  If we block out  the last page [0x3ffff000..0x3fffffff] from
what is available, there is no 15 page span that can happen across the
0x40000000 boundary.  For SG, there can be no merge that connects a
page from one region to another region.  Can you give an example of
the scenario you are thinking of?