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

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

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);
+
                scanline = intel_get_crtc_scanline(crtc);
                if (scanline < min || scanline > max)
                        break;
@@ -145,8 +147,8 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
        }
 
        finish_wait(wq, &wait);
-
-       drm_crtc_vblank_put(&crtc->base);
+       if (have_vblank_irq)
+               drm_crtc_vblank_put(&crtc->base);
 
        /*
         * On VLV/CHV DSI the scanline counter would appear to

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