[Intel-gfx] [PATCH rdma-next v3 1/2] lib/scatterlist: Add support in dynamic allocation of SG table from pages

[Jason Gunthorpe]

On Fri, Sep 25, 2020 at 12:41:29PM +0100, Tvrtko Ursulin wrote:
> 
> On 25/09/2020 08:13, Leon Romanovsky wrote:
> > On Thu, Sep 24, 2020 at 09:21:20AM +0100, Tvrtko Ursulin wrote:
> > > 
> > > On 22/09/2020 09:39, Leon Romanovsky wrote:
> > > > From: Maor Gottlieb <maorg at mellanox.com>
> > > > 
> > > > Extend __sg_alloc_table_from_pages to support dynamic allocation of
> > > > SG table from pages. It should be used by drivers that can't supply
> > > > all the pages at one time.
> > > > 
> > > > This function returns the last populated SGE in the table. Users should
> > > > pass it as an argument to the function from the second call and forward.
> > > > As before, nents will be equal to the number of populated SGEs (chunks).
> > > 
> > > So it's appending and growing the "list", did I get that right? Sounds handy
> > > indeed. Some comments/questions below.
> > 
> > Yes, we (RDMA) use this function to chain contiguous pages.
> 
> I will eveluate if i915 could start using it. We have some loops which build
> page by page and coalesce.

Christoph H doesn't like it, but if there are enough cases we should
really have a pin_user_pages_to_sg() rather than open code this all
over the place.

With THP the chance of getting a coalescing SG is much higher, and
everything is more efficient with larger SGEs.

Jason