[Intel-gfx] [PATCH 02/18] drm/i915/ringbuffer: Brute force context restore

[Chris Wilson]

Quoting Mika Kuoppala (2018-06-08 14:52:13)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > An issue encountered with switching mm on gen7 is that the GPU likes to
> > hang (with the VS unit busy) when told to force restore the current
> > context. We can simply workaround this by substituting the
> > MI_FORCE_RESTORE flag with a round-trip through the kernel_context,
> > forcing the context to be saved and restored; thereby reloading the
> > PP_DIR registers and updating the modified page directory!
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Cc: Matthew Auld <matthew.william.auld at gmail.com>
> > ---
> >  drivers/gpu/drm/i915/intel_ringbuffer.c | 30 ++++++++++++++++++++++---
> >  1 file changed, 27 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > index 65811e2fa7da..332d97bc5c27 100644
> > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > @@ -1458,6 +1458,7 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
> >               (HAS_LEGACY_SEMAPHORES(i915) && IS_GEN7(i915)) ?
> >               INTEL_INFO(i915)->num_rings - 1 :
> >               0;
> > +     bool force_restore = false;
> >       int len;
> >       u32 *cs;
> >  
> > @@ -1471,6 +1472,12 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
> >       len = 4;
> >       if (IS_GEN7(i915))
> >               len += 2 + (num_rings ? 4*num_rings + 6 : 0);
> > +     if (flags & MI_FORCE_RESTORE) {
> > +             GEM_BUG_ON(flags & MI_RESTORE_INHIBIT);
> > +             flags &= ~MI_FORCE_RESTORE;
> > +             force_restore = true;
> > +             len += 2;
> > +     }
> >  
> >       cs = intel_ring_begin(rq, len);
> >       if (IS_ERR(cs))
> > @@ -1496,6 +1503,20 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
> >       }
> >  
> >       *cs++ = MI_NOOP;
> > +     if (force_restore) {
> > +             /*
> > +              * The HW doesn't handle being told to restore the current
> > +              * context very well. Quite often it likes goes to go off and
> > +              * sulk, especially when it is meant to be reloading PP_DIR.
> > +              * A very simple fix to force the reload is to simply switch
> > +              * away from the current context and back again.
> > +              */
> > +             *cs++ = MI_SET_CONTEXT;
> > +             *cs++ = i915_ggtt_offset(to_intel_context(i915->kernel_context,
> > +                                                       engine)->state) |
> > +                     MI_MM_SPACE_GTT |
> > +                     MI_RESTORE_INHIBIT;
> 
> Why inhibit? You dont really switch to kernel but rather overwrite
> current with kernel ctx.

This is for the switch to kernel context. We don't want to load the
kernel context image, just trigger the load of our own context in the
next MI_SET_CONTEXT.

Ideally, we would tell it not to save the kernel context either. But we
don't have that option in MI_SET_CONTEXT. This does mean that the kernel
context then contains state from current, but we *never* use the kernel
context state.
-Chris