drm/i915: Add belts and suspenders locking for seamless M/N changes

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.

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.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230310235828.17439-2-ville.syrjala@linux.intel.com

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 7305ad6..4fd64dc 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -5907,6 +5907,8 @@ 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);
 
@@ -5914,11 +5916,28 @@ 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;
 
        /*
@@ -5962,6 +5981,9 @@ 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);
 }
 
 /*