[PATCH 4/4] drm/atomic-helpers: Make encoder picking more robust

[Daniel Vetter <daniel.vetter@ffwll.ch>]

We've had a few issues with atomic where subtle bugs in the encoder
picking logic lead to accidental self-stealing of the encoder,
resulting in a NULL connector_state->crtc in update_connector_routing
and subsequent.

Linus applied some duct-tape for an mst regression in

commit 27667f4744fc5a0f3e50910e78740bac5670d18b
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed Jul 29 22:18:16 2015 -0700

    i915: temporary fix for DP MST docking station NULL pointer dereference

But that was incomplete (the code will still oops when debuggin is
enabled) and mangled the state even further. So instead WARN and bail
out as the more future-proof option.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 drivers/gpu/drm/drm_atomic_helper.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 8694ca9beee3..9dcc7280e572 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -234,13 +234,14 @@ update_connector_routing(struct drm_atomic_state *state, int conn_idx)
 		}
 	}
 
+	if (WARN_ON(!connector_state->crtc))
+		return -EINVAL;
+
 	connector_state->best_encoder = new_encoder;
-	if (connector_state->crtc) {
-		idx = drm_crtc_index(connector_state->crtc);
+	idx = drm_crtc_index(connector_state->crtc);
 
-		crtc_state = state->crtc_states[idx];
-		crtc_state->mode_changed = true;
-	}
+	crtc_state = state->crtc_states[idx];
+	crtc_state->mode_changed = true;
 
 	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] using [ENCODER:%d:%s] on [CRTC:%d]\n",
 			 connector->base.id,
-- 
2.5.0

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