Re: [PATCH v2] drm/i915: Replace kmap() with kmap_local_page()

[Tvrtko Ursulin]

On 21/06/2023 19:51, Thomas Hellström (Intel) wrote:
> On 6/21/23 18:35, Ira Weiny wrote:
> > Thomas Hellström (Intel) wrote:
> > > I think one thing worth mentioning in the context of this patch is that
> > > IIRC kmap_local_page() will block offlining of the mapping CPU until
> > > kunmap_local(), so while I haven't seen any guidelines around the usage
> > > of this api for long-held mappings, I figure it's wise to keep the
> > > mapping duration short, or at least avoid sleeping with a
> > > kmap_local_page() map active.
> > >
> > > I figured, while page compression is probably to be considered "slow"
> > > it's probably not slow enough to motivate kmap() instead of
> > > kmap_local_page(), but if anyone feels differently, perhaps it should be
> > > considered.
> > What you say is all true.  But remember the mappings are only actually
> > created on a HIGHMEM system.  HIGHMEM systems are increasingly rare.  
> > Also
> > they must suffer such performance issues because there is just no other
> > way around supporting them.
> >
> > Also Sumitra, and our kmap conversion project in general, is focusing on
> > not using kmap* if at all possible.  Thus the reason V1 tried to use
> > page_address().
> >
> > Could we guarantee the i915 driver is excluded from all HIGHMEM systems?
>
> The i915 maintainers might want to chime in here, but I would say no, we 
> can't, although we don't care much about optimizing for them. Same for 
> the new xe driver.

AFAIK i915 works on such systems so I don't think we can drop support 
just like that. Not sure what the process would be. Perhaps as part of a 
wider kernel deprecation would make most sense.

Regards,

Tvrtko