Re: [RFC 5/8] scatterlist: Modify SG copy functions to support io memory.

[Dan Williams]

On Mon, Apr 3, 2017 at 2:20 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
> Hi Christoph,
>
> What are your thoughts on an approach like the following untested
> draft patch.
>
> The patch (if fleshed out) makes it so iomem can be used in an sgl
> and WARN_ONs will occur in places where drivers attempt to access
> iomem directly through the sgl.
>
> I'd also probably create a p2pmem_alloc_sgl helper function so driver
> writers wouldn't have to mess with sg_set_iomem_page.
>
> With all that in place, it should be relatively safe for drivers to
> implement p2pmem even though we'd still technically be violating the
> __iomem boundary in some places.

Just reacting to this mail, I still haven't had a chance to take a
look at the rest of the series.

The pfn_t type was invented to carry extra type and page lookup
information about the memory behind a given pfn. At first glance that
seems a more natural place to carry an indication that this is an
"I/O" pfn.