drm/i915: Add lvds_channel module option

Add a new module optoin lvds_channel to specify the LVDS channel mode
explicitly instead of probing the LVDS register value set by BIOS.
This will be helpful when VBT is broken or incompatible with the
current code.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=42842
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -80,6 +80,12 @@ MODULE_PARM_DESC(lvds_downclock,
 		"Use panel (LVDS/eDP) downclocking for power savings "
 		"(default: false)");
 
+int i915_lvds_channel_mode __read_mostly;
+module_param_named(lvds_channel_mode, i915_lvds_channel_mode, int, 0600);
+MODULE_PARM_DESC(lvds_channel_mode,
+		 "Specify LVDS channel mode "
+		 "(0=probe BIOS [default], 1=single-channel, 2=dual-channel)");
+
 int i915_panel_use_ssc __read_mostly = -1;
 module_param_named(lvds_use_ssc, i915_panel_use_ssc, int, 0600);
 MODULE_PARM_DESC(lvds_use_ssc,

drivers/gpu/drm/i915/i915_drv.h

@@ -1064,6 +1064,7 @@ extern int i915_panel_ignore_lid __read_mostly;
 extern unsigned int i915_powersave __read_mostly;
 extern int i915_semaphores __read_mostly;
 extern unsigned int i915_lvds_downclock __read_mostly;
+extern int i915_lvds_channel_mode __read_mostly;
 extern int i915_panel_use_ssc __read_mostly;
 extern int i915_vbt_sdvo_panel_type __read_mostly;
 extern int i915_enable_rc6 __read_mostly;

drivers/gpu/drm/i915/intel_display.c

@@ -365,6 +365,10 @@ static bool is_dual_link_lvds(struct drm_i915_private *dev_priv,
 {
 	unsigned int val;
 
+	/* use the module option value if specified */
+	if (i915_lvds_channel_mode > 0)
+		return i915_lvds_channel_mode == 2;
+
 	if (dev_priv->lvds_val)
 		val = dev_priv->lvds_val;
 	else {