[Intel-gfx] [PATCH 1/6] lib/scatterlist: add sg_set_dma_addr() function

[Christoph Hellwig]

On Thu, Mar 12, 2020 at 11:19:28AM -0300, Jason Gunthorpe wrote:
> The non-page scatterlist is also a big concern for RDMA as we have
> drivers that want the page list, so even if we did as this series
> contemplates I'd have still have to split the drivers and create the
> notion of a dma-only SGL.

The drivers I looked at want a list of IOVA address, aligned to the
device "page size".  What other data do drivers want?  Execept for the
software protocol stack drivers, which of couse need pages for the
stack futher down.

> I haven't used bio_vecs before, do they support chaining like SGL so
> they can be very big? RDMA dma maps gigabytes of memory

bio_vecs itself don't have the chaining, but the bios build around them
do.  But each entry can map a huge pile.  If needed we could use the
same chaining scheme we use for scatterlists for bio_vecs as well, but
lets see if we really end up needing that.

> So I'm guessing the path forward is something like
> 
>  - Add some generic dma_sg data structure and helper
>  - Add dma mapping code to go from pages to dma_sg

That has been on my todo list for a while.  All the DMA consolidatation
is to prepare for that and we're finally getting close.