Re: [PATCH 1/5] drm/i915: Keep vblank irq enabled during vblank evasion.

Chris Wilson
2018-02-12T20:55:10Z

Quoting Ville Syrjälä (2018-02-12 18:06:58)
> On Mon, Feb 12, 2018 at 05:24:54PM +0000, Chris Wilson wrote:
> > Quoting Ville Syrjälä (2018-02-12 16:55:28)
> > > On Mon, Feb 12, 2018 at 04:41:05PM +0100, Maarten Lankhorst wrote:
> > > > Op 12-02-18 om 16:31 schreef Chris Wilson:
> > > > > Quoting Maarten Lankhorst (2018-02-12 15:27:34)
> > > > >> Op 12-02-18 om 16:22 schreef Chris Wilson:
> > > > >>> Quoting Maarten Lankhorst (2018-02-12 15:16:39)
> > > > >>>> Op 12-02-18 om 16:10 schreef Chris Wilson:
> > > > >>>>> Quoting Maarten Lankhorst (2018-02-09 09:54:00)
> > > > >>>>>> This is a nice preparation for grabbing the uncore lock during evasion.
> > > > >>>>>> Grabbing the spinlock with the lock held messes up the locking,
> > > > >>>>>> so it's easier to handover the reference to the eve
> > > > >>>>>>
> > > > >>>>>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > > >>>>>> ---
> > > > >>>>>> drivers/gpu/drm/i915/intel_sprite.c | 11 ++++-------
> > > > >>>>>> 1 file changed, 4 insertions(+), 7 deletions(-)
> > > > >>>>>>
> > > > >>>>>> diff --git a/drivers/gpu/drm/i915/intel_sprite.c b/drivers/gpu/drm/i915/intel_sprite.c
> > > > >>>>>> index 3be22c0fcfb5..971a1ea0db45 100644
> > > > >>>>>> --- a/drivers/gpu/drm/i915/intel_sprite.c
> > > > >>>>>> +++ b/drivers/gpu/drm/i915/intel_sprite.c
> > > > >>>>>> @@ -109,10 +109,10 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
> > > > >>>>>>
> > > > >>>>>> local_irq_disable();
> > > > >>>>>>
> > > > >>>>>> - if (min <= 0 || max <= 0)
> > > > >>>>>> + if (WARN_ON(drm_crtc_vblank_get(&crtc->base)))
> > > > >>>>>> return;
> > > > >>>>>>
> > > > >>>>>> - if (WARN_ON(drm_crtc_vblank_get(&crtc->base)))
> > > > >>>>>> + if (min <= 0 || max <= 0)
> > > > >>>>>> return;
> > > > >>>>>>
> > > > >>>>> The corresponding vblank_put is the one later in update_start(), right?
> > > > >>>>> I don't think you intended to keep this chunk.
> > > > >>>>> -Chris
> > > > >>>> I'm not sure what you mean? The vblank_put is now in pipe_update_end, except if the
> > > > >>>> event takes over the reference. I think the code is correct. :)
> > > > >>> Then it's unbalanced in the case of error still.
> > > > >>> -Chris
> > > > >> It already would have been for events, hence the WARN_ON there.
> > > > >> I don't think we can do anything about it, this shouldn't ever
> > > > >> happen in practice, could be a BUG_ON for all I care. :)
> > > > > I would much prefer that over intentionally bad code.
> > > > >
> > > > > But do we really need to enable the vblank irq here? If the event
> > > > > requires it, doesn't it already enable the vblank. Here, we only need it
> > > > > when sleeping, can we not determine we have enough time before the
> > > > > vblank without enabling the interrupt?
> > > > I'm not sure why we get a reference to the vblank counter here. Perhaps Ville does?
> > >
> > > We need the vblank irq to be enabled before we check the scanline since
> > > otherwise we may end up doing:
> > >
> > > 1. check scanline
> > > 3. vblank irq fires
> > > 2. enable vblank irq
> > > 3. wait for the next vblank
> > >
> > > So we'd end up wasting an entire frame.
> >
> > Step: 2.5, check_scanline?
> >
> > Something like,
> >
> > diff --git a/drivers/gpu/drm/i915/intel_sprite.c b/drivers/gpu/drm/i915/intel_sprite.c
> > index 574bd02c5a2e..70c2ee1c7b8c 100644
> > --- a/drivers/gpu/drm/i915/intel_sprite.c
> > +++ b/drivers/gpu/drm/i915/intel_sprite.c
> > @@ -97,6 +97,7 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
> > bool need_vlv_dsi_wa = (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv)) &&
> > intel_crtc_has_type(new_crtc_state, INTEL_OUTPUT_DSI);
> > DEFINE_WAIT(wait);
> > + bool have_vblank_irq = false;
> >
> > vblank_start = adjusted_mode->crtc_vblank_start;
> > if (adjusted_mode->flags & DRM_MODE_FLAG_INTERLACE)
> > @@ -112,9 +113,6 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
> > if (min <= 0 || max <= 0)
> > return;
> >
> > - if (WARN_ON(drm_crtc_vblank_get(&crtc->base)))
> > - return;
> > -
> > crtc->debug.min_vbl = min;
> > crtc->debug.max_vbl = max;
> > trace_i915_pipe_update_start(crtc);
> > @@ -127,6 +125,10 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
> > */
> > prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
> >
> > + if (!have_vblank_irq)
> > + have_vblank_irq = !drm_crtc_vblank_get(&crtc->base);
> > +
>
> This doesn't seem to change anything.

Nothing wrt to the existing code :)

The idea is that we have to enable the vblank-irq before doing the
scanline check in order to not miss the interrupt, which as I understand
it was the danger you highlighted with trying to avoid taking the
vblank-irq.
-Chris