Re: [Intel-gfx] [PATCH 2/4] drm/i915: Add belts and suspenders locking for seamless M/N changes

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 07, 2023 at 02:24:08PM +0200, Jani Nikula wrote:
> On Mon, 06 Mar 2023, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > Add some (probably overkill) locking to protect the vblank
> > timestamping constants updates during seamless M/N fastsets.
> >
> > As everything should be naturally aligned I think the individual
> > pieces should probably end up updating atomically enough. So this
> > is only really meant to guarantee everyone sees a consistent whole.
> >
> > All the drm_vblank.c usage is covered by vblank_time_lock,
> > and uncore.lock will take care of __intel_get_crtc_scanline()
> > that can also be called from outside the core vblank functionality.
> 
> The patch seems to do what it says on the box, but I increasingly
> dislike the use of uncore.lock for anything other than the nuts and
> bolts of uncore.

Yeah, it's not really great. Hence the TODO I left behind there.

> 
> BR,
> Jani.
> 
> >
> > Currently only crtc_clock and framedur_ns can change, but in
> > the future might fastset also across eg. vtotal/vblank_end
> > changes, so let's just grab the locks across the whole thing.
> >
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_display.c | 24 +++++++++++++++++++-
> >  1 file changed, 23 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> > index a1fbdf32bd21..020320468967 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > @@ -5908,6 +5908,8 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state)
> >  	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
> >  	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
> >  	struct drm_display_mode adjusted_mode;
> > +	int vmax_vblank_start = 0;
> > +	unsigned long irqflags;
> >  
> >  	drm_mode_init(&adjusted_mode, &crtc_state->hw.adjusted_mode);
> >  
> > @@ -5915,11 +5917,28 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state)
> >  		adjusted_mode.crtc_vtotal = crtc_state->vrr.vmax;
> >  		adjusted_mode.crtc_vblank_end = crtc_state->vrr.vmax;
> >  		adjusted_mode.crtc_vblank_start = intel_vrr_vmin_vblank_start(crtc_state);
> > -		crtc->vmax_vblank_start = intel_vrr_vmax_vblank_start(crtc_state);
> > +		vmax_vblank_start = intel_vrr_vmax_vblank_start(crtc_state);
> >  	}
> >  
> > +	/*
> > +	 * Belts and suspenders locking to guarantee everyone sees 100%
> > +	 * consistent state during fastset seamless refresh rate changes.
> > +	 *
> > +	 * vblank_time_lock takes care of all drm_vblank.c stuff, and
> > +	 * uncore.lock takes care of __intel_get_crtc_scanline() which
> > +	 * may get called elsewhere as well.
> > +	 *
> > +	 * TODO maybe just protect everything (including
> > +	 * __intel_get_crtc_scanline()) with vblank_time_lock?
> > +	 * Need to audit everything to make sure it's safe.
> > +	 */
> > +	spin_lock_irqsave(&dev_priv->drm.vblank_time_lock, irqflags);
> > +	spin_lock(&dev_priv->uncore.lock);
> > +
> >  	drm_calc_timestamping_constants(&crtc->base, &adjusted_mode);
> >  
> > +	crtc->vmax_vblank_start = vmax_vblank_start;
> > +
> >  	crtc->mode_flags = crtc_state->mode_flags;
> >  
> >  	/*
> > @@ -5963,6 +5982,9 @@ void intel_crtc_update_active_timings(const struct intel_crtc_state *crtc_state)
> >  	} else {
> >  		crtc->scanline_offset = 1;
> >  	}
> > +
> > +	spin_unlock(&dev_priv->uncore.lock);
> > +	spin_unlock_irqrestore(&dev_priv->drm.vblank_time_lock, irqflags);
> >  }
> >  
> >  /*
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center

-- 
Ville Syrjälä
Intel