[PATCH 4/4] drm/i915/display: Set TRANS_DDI_MODE_SELECT to default value when disabling TRANS_DDI

[José Roberto de Souza]

TGL timeouts when disabling MST transcoder and fifo underruns over MST
transcoders are fixed when setting TRANS_DDI_MODE_SELECT to 0(HDMI
mode) during the disable sequence.

Although BSpec disable sequence don't require this step it is a
harmless change and it is also done by Windows driver.
Anyhow HW team was notified about that but it can take some time to
documentation to be updated.

A case that always lead to those issues is:
- do a modeset enabling pipe A and pipe B in the same MST stream
leaving A as master
- disable pipe A, promote B as master doing a full modeset in A
- enable pipe A, changing the master transcoder back to A(doing a
full modeset in B)
- Pow: underruns and timeouts

The transcoders involved will only work again when complete disabled
and their power wells turned off causing a reset in their registers.

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Matt Roper <matthew.d.roper at intel.com>
Signed-off-by: José Roberto de Souza <jose.souza at intel.com>
---
 drivers/gpu/drm/i915/display/intel_ddi.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
index 32ea3c7e8b62..82e90f271974 100644
--- a/drivers/gpu/drm/i915/display/intel_ddi.c
+++ b/drivers/gpu/drm/i915/display/intel_ddi.c
@@ -1997,6 +1997,7 @@ void intel_ddi_disable_transcoder_func(const struct intel_crtc_state *crtc_state
 
 	val = I915_READ(TRANS_DDI_FUNC_CTL(cpu_transcoder));
 	val &= ~TRANS_DDI_FUNC_ENABLE;
+	val &= ~TRANS_DDI_MODE_SELECT_MASK;
 
 	if (INTEL_GEN(dev_priv) >= 12) {
 		if (!intel_dp_mst_is_master_trans(crtc_state))
-- 
2.25.0