From 29b27657dbae8d4c4bdc6bfbf6dd8c69f648e075 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@linux.intel.com>
Date: Wed, 2 Oct 2019 19:25:05 +0300
Subject: drm/i915/mst: Document the userspace fail with possible_crtcs
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Git-commit: 29b27657dbae8d4c4bdc6bfbf6dd8c69f648e075
Patch-mainline: v5.5-rc1
References: bsc#1152489
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.
v2: Drop a bogus TODO comment
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002162505.30716-6-ville.syrjala@linux.intel.com
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
drivers/gpu/drm/i915/display/intel_dp_mst.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index 7370c9dfa8a1..715b7109c388 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
@@ -615,6 +615,14 @@ 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 :(
+ */
intel_encoder->pipe_mask = ~0;
intel_encoder->compute_config = intel_dp_mst_compute_config;
--
2.28.0