[Intel-gfx] [PATCH 6/6] drm/i915/mst: Document the userspace fail with possible_crtcs

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

To avoid accidentally breaking things in the future add a
comment explaining why we misconfigure the pipe_mask.

Also toss in a TODO for investigating a single encoder
approach as opposed to the encoder-per-pipe approach.

Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp_mst.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index 7be82cf926ca..cb3047fe2d02 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
@@ -615,6 +615,18 @@ intel_dp_create_fake_mst_encoder(struct intel_digital_port *intel_dig_port, enum
 	intel_encoder->power_domain = intel_dig_port->base.power_domain;
 	intel_encoder->port = intel_dig_port->base.port;
 	intel_encoder->cloneable = 0;
+	/*
+	 * This is wrong, but broken userspace uses the intersection
+	 * of possible_crtcs of all the encoders of a given connector
+	 * to figure out which crtcs can drive said connector. What
+	 * should be used instead is the union of possible_crtcs.
+	 * To keep such userspace functioning we must misconfigure
+	 * this to make sure the intersection is not empty :(
+	 *
+	 * TODO: figure out if we could eliminate the per-pipe
+	 * encoders here and just have a single encoder for each
+	 * MST connector. That would sidestep the userspace bug.
+	 */
 	intel_encoder->pipe_mask = ~0;
 
 	intel_encoder->compute_config = intel_dp_mst_compute_config;
-- 
2.21.0