[PATCH 1/4] drm/amd/display: Prevent vblank irq disable while VRR is active.

[Mario Kleiner <mario.kleiner.de@gmail.com>]

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.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 35 +++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index a718ac2..c1c3815 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/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;
@@ -4716,6 +4722,32 @@ static void update_freesync_state_on_stream(
 			      (int)vrr_params.state);
 }
 
+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,
@@ -4757,6 +4789,9 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 		goto cleanup;
 	}
 
+	/* Take care to hold extra vblank ref for a crtc in VRR mode */
+	amdgpu_dm_handle_vrr_transition(dm_old_crtc_state, acrtc_state);
+
 	/* update planes when needed */
 	for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
 		struct drm_crtc *crtc = new_plane_state->crtc;
-- 
2.7.4

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