Re: [PATCH] drm/i915: Assert we hold the CRTC powerwell for generating vblank interrupts

Ville Syrjälä
2016-10-11T06:55:42Z

On Tue, Oct 11, 2016 at 08:17:22AM +0200, Maarten Lankhorst wrote:
> Op 10-10-16 om 13:56 schreef Ville Syrjälä:
> > On Mon, Oct 10, 2016 at 12:46:32PM +0100, Chris Wilson wrote:
> >> On Mon, Oct 10, 2016 at 02:42:01PM +0300, Ville Syrjälä wrote:
> >>> On Mon, Oct 10, 2016 at 12:34:54PM +0100, Chris Wilson wrote:
> >>>> To enable the vblank itself, we need to have an RPM wakeref for the mmio
> >>>> access, and whilst generating the vblank interrupts we continue to
> >>>> require the rpm wakeref. The assumption is that the RPM wakeref is held
> >>>> by the display powerwell held by the active pipe. As this chain was not
> >>>> obvious to me chasing the drm_wait_vblank ioctl, document it with a WARN
> >>>> during *_vblank_enable().
> >>>>
> >>>> v2: Check the display power well rather than digging inside the atomic
> >>>> CRTC state.
> >>>>
> >>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> >>>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >>>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> >>>> ---
> >>>> drivers/gpu/drm/i915/i915_irq.c | 20 ++++++++++++++++++++
> >>>> 1 file changed, 20 insertions(+)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> >>>> index 1e43fe30da11..f0f17055dbb9 100644
> >>>> --- a/drivers/gpu/drm/i915/i915_irq.c
> >>>> +++ b/drivers/gpu/drm/i915/i915_irq.c
> >>>> @@ -2715,6 +2715,14 @@ void i915_handle_error(struct drm_i915_private *dev_priv,
> >>>> i915_reset_and_wakeup(dev_priv);
> >>>> }
> >>>>
> >>>> +static void assert_pipe_is_awake(struct drm_i915_private *dev_priv,
> >>>> + enum pipe pipe)
> >>>> +{
> >>>> + WARN_ON(IS_ENABLED(CONFIG_DRM_I915_DEBUG) &&
> >>>> + !intel_display_power_is_enabled(dev_priv,
> >>>> + POWER_DOMAIN_PIPE(pipe)));
> >>> Uses a mutex. And having a power well enabled doesn't mean much. It
> >>> doesn't guarantee that vblanks work.
> >> Impasse. :|
> >>
> >> There should be no point in an explicit assert_rpm_wakeref here as the
> >> register access should catch an error there. Is there no safe way we can
> >> assert the current state of the CRTC is correct for enabling vblanks?
> > crtc->active might be the closest thing, if we just ignore any locking.
> > Though it looks like that has gone a bit mad these days, what with being
> > set from the .crtc_enable() hooks but getting cleared outside the
> > .crtc_disable() hooks.
> >
> I'm trying to kill crtc->active.

Because it's evil? I still don't see much problem in having a thing to
track the state of each pipe fairly accurately.

> Maybe you'd want to use dev_priv->active_crtcs, but that won't save you if you enable interrupts in between atomic commit and .crtc_disable

Nothing atomic based will work well because the state is not flipped at
the same time as the actual hardware state changes.

>
> Safest bet is to look at the power state I think.

I don't know which power state you mean, but I already shot down the
power domain thing.

--
Ville Syrjälä
Intel OTC