Re: [PATCH] drm/i915: Release the atomic kmap relocation cache around snb GTT w/a

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2018-02-13 13:42:03)
> 
> On 12/02/2018 21:11, Chris Wilson wrote:
> > When we need to rebind the vma into the global GTT for snb, we need to
> > drop the current reloc_cache as it will be holding a kmap_atomic() and
> > we may need to sleep for i915_vma_bind(). In practice, this is not an
> > issue as we already hold an rpm reference for the execbuffer, but with
> > tighter error checking around rpm we need to be more careful.
> > 
> > References: 31a39207f04a ("drm/i915: Cache kmap between relocations")
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_gem_execbuffer.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > index b15305f2fb76..8c34b1b5a126 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > @@ -1315,6 +1315,12 @@ eb_relocate_entry(struct i915_execbuffer *eb,
> >                */
> >               if (reloc->write_domain == I915_GEM_DOMAIN_INSTRUCTION &&
> >                   IS_GEN6(eb->i915)) {
> > +                     /*
> > +                      * Release the kmap_atomic cache in order to allow the
> > +                      * i915_vma_bind() to sleep (if needs be).
> > +                      */
> > +                     reloc_cache_reset(&eb->reloc_cache);
> > +
> >                       err = i915_vma_bind(target, target->obj->cache_level,
> >                                           PIN_GLOBAL);
> >                       if (WARN_ONCE(err,
> > 
> 
> Hmm yes, very interesting. If you are happy with the performance hit then:
> 
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> But in general could it also be solved by changing how 
> intel_runtime_pm_get works - making our wakeref_count top level and only 
> calling pm_runtime_get_sync on 0 to 1 transition? That would solve the 
> issue with proposed might_sleep and code paths which know the condition 
> is actually impossible.

I think we should do that anyway to reduce the jitter we see in calling
pm_runtime_get_sync().
-Chris
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