preserve DMA offsets when using swiotlb v2

[Christoph Hellwig]

Sorry for being a little pushy, any chance we could get this reviewed
in time for the 5.12 merge window?

On Sun, Feb 07, 2021 at 05:03:18PM +0100, Christoph Hellwig wrote:
> Hi all,
> 
> this series make NVMe happy when running with swiotlb.  This caters
> towards to completely broken NVMe controllers that ignore the
> specification (hello to the biggest cloud provider on the planet!),
> to crappy SOC that have addressing limitations, or "secure"
> virtualization that force bounce buffering to enhance the user
> experience.  Or in other words, no one sane should hit it, but
> people do.
> 
> It is basically a respin of the
> 
>     "SWIOTLB: Preserve swiotlb map offset when needed."
> 
> series from Jianxiong Gao.  It complete rewrites the swiotlb part so that
> the offset really is preserved and not just the offset into the swiotlb
> slot, and to do so it grew half a dozen patches to refactor the swiotlb
> so that a mere mortal like me could actually understand it.
> 
> Changes since v2:
>  - cleanup nr_slots a little more
>  - fix a check in get_max_slots
>  - add a IO_TLB_SIZE define
>  - cleanup find_slots a little more
>  - fix the nvme patch to actually compile
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