[PATCH v2] drm/i915/ddi: Guard reg_val against a INVALID_TRANSCODER

[Suraj Kandpal]

Currently we check if the encoder is INVALID or -1 and throw a
WARN_ON but we still end up writing the temp value which will
overflow and corrupt the whole programmed value.

--v2
-Assign a bogus transcoder to master in case we get a INVALID
TRANSCODER [Jani]

Fixes: 6671c367a9bea ("drm/i915/tgl: Select master transcoder for MST stream")
Signed-off-by: Suraj Kandpal <suraj.kand...@intel.com>
---
 drivers/gpu/drm/i915/display/intel_ddi.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c 
b/drivers/gpu/drm/i915/display/intel_ddi.c
index 46017091bb0b..f67ce35c1c00 100644
--- a/drivers/gpu/drm/i915/display/intel_ddi.c
+++ b/drivers/gpu/drm/i915/display/intel_ddi.c
@@ -596,8 +596,9 @@ intel_ddi_transcoder_func_reg_val_get(struct intel_encoder 
*encoder,
                        enum transcoder master;
 
                        master = crtc_state->mst_master_transcoder;
-                       drm_WARN_ON(display->drm,
-                                   master == INVALID_TRANSCODER);
+                       if (drm_WARN_ON(display->drm,
+                                       master == INVALID_TRANSCODER))
+                               master = TRANSCODER_A;
                        temp |= TRANS_DDI_MST_TRANSPORT_SELECT(master);
                }
        } else {
-- 
2.34.1