drm: Change limited M/N quirk to constant N quirk.

Some DP dongles in particular seem to be fussy about too large
link M/N values. Set specific value for N divider can resolve
this issue per dongle vendor's comment. So configure N as
constant value (0x8000) to instead of reduce M/N formula when
specific DP dongle connected.

v2: add more comments for issue description and fix typo.
v3: add lost commit messages back for version 2
v4: send patch to both intel-gfx and dri-devel

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Tested-by: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Lee, Shawn C <shawn.c.lee@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1536733371-25004-3-git-send-email-shawn.c.lee@intel.com

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index 7e3e01607643..b7505879b1f7 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -45,8 +45,8 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
         int lane_count, slots;
         const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
         int mst_pbn;
-        bool reduce_m_n = drm_dp_has_quirk(&intel_dp->desc,
-                                           DP_DPCD_QUIRK_LIMITED_M_N);
+        bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
+                                           DP_DPCD_QUIRK_CONSTANT_N);
 
         if (adjusted_mode->flags & DRM_MODE_FLAG_DBLSCAN)
                 return false;
@@ -87,7 +87,7 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
                                adjusted_mode->crtc_clock,
                                pipe_config->port_clock,
                                &pipe_config->dp_m_n,
-                               reduce_m_n);
+                               constant_n);
 
         pipe_config->dp_m_n.tu = slots;