drm/i915: Assume dual channel LVDS if pixel clock necessitates it

Single channel LVDS maxes out at 112 MHz, anything above must be dual
channel. This avoids the need to specify i915.lvds_channel_mode=2 on
all 17" MacBook Pro models with i915 graphics since they had 1920x1200
(193 MHz), plus those 15" pre-retina models which had a resolution
of 1680x1050 (119 MHz) as a BTO option.

Source for 112 MHz limit of single channel LVDS is section 2.3 of:
https://01.org/linuxgraphics/sites/default/files/documentation/ivb_ihd_os_vol3_part4.pdf

v2: Avoid hardcoding 17" models by assuming dual channel LVDS if the
resolution necessitates it, suggested by Jani Nikula.

v3: Fix typo, thanks Joonas Lahtinen.

v4: Split commit in two, suggested by Ville Syrjälä.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Tested-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
[Jani: included spec reference into the commit message]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_lvds.c b/drivers/gpu/drm/i915/intel_lvds.c
index 5abda1d2c018..ee621e335d04 100644
--- a/drivers/gpu/drm/i915/intel_lvds.c
+++ b/drivers/gpu/drm/i915/intel_lvds.c
@@ -848,6 +848,11 @@ static bool compute_is_dual_link_lvds(struct intel_lvds_encoder *lvds_encoder)
         if (i915.lvds_channel_mode > 0)
                 return i915.lvds_channel_mode == 2;
 
+        /* single channel LVDS is limited to 112 MHz */
+        if (lvds_encoder->attached_connector->base.panel.fixed_mode->clock
+            > 112999)
+                return true;
+
         if (dmi_check_system(intel_dual_link_lvds))
                 return true;
 
@@ -1111,6 +1116,8 @@ void intel_lvds_init(struct drm_device *dev)
 out:
         mutex_unlock(&dev->mode_config.mutex);
 
+        intel_panel_init(&intel_connector->panel, fixed_mode, downclock_mode);
+
         lvds_encoder->is_dual_link = compute_is_dual_link_lvds(lvds_encoder);
         DRM_DEBUG_KMS("detected %s-link lvds configuration\n",
                       lvds_encoder->is_dual_link ? "dual" : "single");
@@ -1125,7 +1132,6 @@ out:
         }
         drm_connector_register(connector);
 
-        intel_panel_init(&intel_connector->panel, fixed_mode, downclock_mode);
         intel_panel_setup_backlight(connector, INVALID_PIPE);
 
         return;