drm/i915: Detect USB-C specific dongles before reducing M and N

The Analogix 7737 DP to HDMI converter requires reduced M and N values
when to operate correctly at HBR2. We tried to reduce the M/N values for
all devices in commit 9a86cda07af2 ("drm/i915/dp: reduce link M/N
parameters"), but that regressed some other sinks. Detect this IC by its
OUI value of 0x0022B9 via the DPCD quirk list, and only reduce the M/N
values for that.

v2 by Jani: Rebased on the DP quirk database

v3 by Jani: Rebased on the reworked DP quirk database

v4 by Jani: Improve commit message (Daniel)

Fixes: 9a86cda07af2 ("drm/i915/dp: reduce link M/N parameters")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93578
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100755
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/2d2e30f8f47d3f28c9b74ca2612336a54585c3ec.1495105635.git.jani.nikula@intel.com

1 files changed, 4 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index c1f62eb07c07..989e25577ac0 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -44,6 +44,8 @@ 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);
 
         pipe_config->has_pch_encoder = false;
         bpp = 24;
@@ -75,7 +77,8 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
         intel_link_compute_m_n(bpp, lane_count,
                                adjusted_mode->crtc_clock,
                                pipe_config->port_clock,
-                               &pipe_config->dp_m_n);
+                               &pipe_config->dp_m_n,
+                               reduce_m_n);
 
         pipe_config->dp_m_n.tu = slots;