drm/i915: add module param "enable_dp_mst"

Adds an (unsafe; auto-kernel-tainting) boolean module parameter to the i915
drm driver: "enable_dp_mst", which is enabled by default.  Disabling the
parameter forces newly connected DisplayPort sinks to report as not
supporting multi-stream transport (MST), thus "forcing" the use of
single-stream transport (SST).

v2: rename parameter to conform to style
v3: add signoff

Signed-off-by: Nathan Schulte <nmschulte@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458054845-5837-1-git-send-email-nmschulte@gmail.com

drivers/gpu/drm/i915/i915_params.c

@@ -56,6 +56,7 @@ struct i915_params i915 __read_mostly = {
 	.edp_vswing = 0,
 	.enable_guc_submission = false,
 	.guc_log_level = -1,
+	.enable_dp_mst = true,
 };
 
 module_param_named(modeset, i915.modeset, int, 0400);
@@ -201,3 +202,7 @@ MODULE_PARM_DESC(enable_guc_submission, "Enable GuC submission (default:false)")
 module_param_named(guc_log_level, i915.guc_log_level, int, 0400);
 MODULE_PARM_DESC(guc_log_level,
 	"GuC firmware logging level (-1:disabled (default), 0-3:enabled)");
+
+module_param_named_unsafe(enable_dp_mst, i915.enable_dp_mst, bool, 0600);
+MODULE_PARM_DESC(enable_dp_mst,
+	"Enable multi-stream transport (MST) for new DisplayPort sinks. (default: true)");

drivers/gpu/drm/i915/i915_params.h

@@ -59,6 +59,7 @@ struct i915_params {
 	bool enable_guc_submission;
 	bool verbose_state_checks;
 	bool nuclear_pageflip;
+	bool enable_dp_mst;
 };
 
 extern struct i915_params i915 __read_mostly;

drivers/gpu/drm/i915/intel_dp.c

@@ -3882,6 +3882,9 @@ intel_dp_probe_mst(struct intel_dp *intel_dp)
 {
 	u8 buf[1];
 
+	if (!i915.enable_dp_mst)
+		return false;
+
 	if (!intel_dp->can_mst)
 		return false;