drm/amd/display: Prevent vblank irq disable while VRR is active. (v3)

During VRR mode we can not allow vblank irq dis-/enable
transitions, as an enable after a disable can happen at
an arbitrary time during the video refresh cycle, e.g.,
with a high likelyhood inside vblank front-porch. An
enable during front-porch would cause vblank timestamp
updates/calculations which are completely bogus, given
the code can't know when the vblank will end as long
as we are in front-porch with no page flip completed.

Hold a permanent vblank reference on the crtc while
in active VRR mode to prevent a vblank disable, and
drop the reference again when switching back to fixed
refresh rate non-VRR mode.

v2: Make sure transition is also handled if vrr is
    disabled and stream gets disabled in the same
    atomic commit by moving the call to the transition
    function outside of plane commit.
    Suggested by Nicholas.

v3: Trivial rebase onto previous patch.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -251,6 +251,12 @@ get_crtc_by_otg_inst(struct amdgpu_device *adev,
 	return NULL;
 }
 
+static inline bool amdgpu_dm_vrr_active(struct dm_crtc_state *dm_state)
+{
+	return dm_state->freesync_config.state == VRR_STATE_ACTIVE_VARIABLE ||
+	       dm_state->freesync_config.state == VRR_STATE_ACTIVE_FIXED;
+}
+
 static void dm_pflip_high_irq(void *interrupt_params)
 {
 	struct amdgpu_crtc *amdgpu_crtc;
@@ -4738,6 +4744,31 @@ static void pre_update_freesync_state_on_stream(
 	new_crtc_state->vrr_params = vrr_params;
 }
 
+static void amdgpu_dm_handle_vrr_transition(struct dm_crtc_state *old_state,
+					    struct dm_crtc_state *new_state)
+{
+	bool old_vrr_active = amdgpu_dm_vrr_active(old_state);
+	bool new_vrr_active = amdgpu_dm_vrr_active(new_state);
+
+	if (!old_vrr_active && new_vrr_active) {
+		/* Transition VRR inactive -> active:
+		 * While VRR is active, we must not disable vblank irq, as a
+		 * reenable after disable would compute bogus vblank/pflip
+		 * timestamps if it likely happened inside display front-porch.
+		 */
+		drm_crtc_vblank_get(new_state->base.crtc);
+		DRM_DEBUG_DRIVER("%s: crtc=%u VRR off->on: Get vblank ref\n",
+				 __func__, new_state->base.crtc->base.id);
+	} else if (old_vrr_active && !new_vrr_active) {
+		/* Transition VRR active -> inactive:
+		 * Allow vblank irq disable again for fixed refresh rate.
+		 */
+		drm_crtc_vblank_put(new_state->base.crtc);
+		DRM_DEBUG_DRIVER("%s: crtc=%u VRR on->off: Drop vblank ref\n",
+				 __func__, new_state->base.crtc->base.id);
+	}
+}
+
 static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 				    struct dc_state *dc_state,
 				    struct drm_device *dev,
@@ -5278,6 +5309,11 @@ static void amdgpu_dm_atomic_commit_tail(struct drm_atomic_state *state)
 
 		dm_new_crtc_state = to_dm_crtc_state(new_crtc_state);
 		dm_old_crtc_state = to_dm_crtc_state(old_crtc_state);
+
+		/* Handle vrr on->off / off->on transitions */
+		amdgpu_dm_handle_vrr_transition(dm_old_crtc_state,
+						dm_new_crtc_state);
+
 		modeset_needed = modeset_required(
 				new_crtc_state,
 				dm_new_crtc_state->stream,