drm/i915/ehl: Allow combo PHY A to drive a third external display

EHL has a mux on combo PHY A that allows it to be driven either by an
internal display (DDI-A or DSI DPHY) or by an external display (DDI-D).
This is a motherboard design decision that can not be changed on the
fly.  Unfortunately there are no strap registers that allow us to detect
the board configuration directly, so let's use the VBT to try to figure
it out and program the mux accordingly.

For now if we run across a broken VBT that tries to claim that PHY A
is attached to both internal and external displays at the same time,
we'll resolve the conflict in favor of the internal display.  To help
debug these kind of bad VBT's, let's also add a quick DRM_DEBUG message
during child device parsing so that it's easier to understand these
cases if they show up in bug reports.

v2:
 - Confirmed that VBT's dvo port refers to the DDI and not the PHY.
   Thus we can check more explicitly for (ddi_d && !(ddi_a || dsi)).  If
   a bad VBT contradicts itself, let internal display win.  (Ville)

v3:
 - Switch condition from !IS_ICELAKE to IS_ELKHARTLAKE.  Although the
   convention is usually to assume that future platforms will inherit
   all current platform behavior, this feels more like a one-platform
   quirk.  (Ville)
 - Update commit message to describe what we do if/when we encounter
   broken VBT's, and note that the new debug print during child device
   parsing is intentional.

Cc: Clint Taylor <Clinton.A.Taylor@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618175131.9139-1-matthew.d.roper@intel.com

drivers/gpu/drm/i915/display/intel_bios.c

@@ -1668,6 +1668,9 @@ parse_general_definitions(struct drm_i915_private *dev_priv,
 		if (!child->device_type)
 			continue;
 
+		DRM_DEBUG_KMS("Found VBT child device with type 0x%x\n",
+			      child->device_type);
+
 		/*
 		 * Copy as much as we know (sizeof) and is available
 		 * (child_dev_size) of the child device. Accessing the data must

drivers/gpu/drm/i915/display/intel_combo_phy.c

@@ -260,6 +260,32 @@ void intel_combo_phy_power_up_lanes(struct drm_i915_private *dev_priv,
 	I915_WRITE(ICL_PORT_CL_DW10(port), val);
 }
 
+static u32 ehl_combo_phy_a_mux(struct drm_i915_private *i915, u32 val)
+{
+	bool ddi_a_present = i915->vbt.ddi_port_info[PORT_A].child != NULL;
+	bool ddi_d_present = i915->vbt.ddi_port_info[PORT_D].child != NULL;
+	bool dsi_present = intel_bios_is_dsi_present(i915, NULL);
+
+	/*
+	 * VBT's 'dvo port' field for child devices references the DDI, not
+	 * the PHY.  So if combo PHY A is wired up to drive an external
+	 * display, we should see a child device present on PORT_D and
+	 * nothing on PORT_A and no DSI.
+	 */
+	if (ddi_d_present && !ddi_a_present && !dsi_present)
+		return val | ICL_PHY_MISC_MUX_DDID;
+
+	/*
+	 * If we encounter a VBT that claims to have an external display on
+	 * DDI-D _and_ an internal display on DDI-A/DSI leave an error message
+	 * in the log and let the internal display win.
+	 */
+	if (ddi_d_present)
+		DRM_ERROR("VBT claims to have both internal and external displays on PHY A.  Configuring for internal.\n");
+
+	return val & ~ICL_PHY_MISC_MUX_DDID;
+}
+
 static void icl_combo_phys_init(struct drm_i915_private *dev_priv)
 {
 	enum port port;
@@ -273,7 +299,17 @@ static void icl_combo_phys_init(struct drm_i915_private *dev_priv)
 			continue;
 		}
 
+		/*
+		 * EHL's combo PHY A can be hooked up to either an external
+		 * display (via DDI-D) or an internal display (via DDI-A or
+		 * the DSI DPHY).  This is a motherboard design decision that
+		 * can't be changed on the fly, so initialize the PHY's mux
+		 * based on whether our VBT indicates the presence of any
+		 * "internal" child devices.
+		 */
 		val = I915_READ(ICL_PHY_MISC(port));
+		if (IS_ELKHARTLAKE(dev_priv) && port == PORT_A)
+			val = ehl_combo_phy_a_mux(dev_priv, val);
 		val &= ~ICL_PHY_MISC_DE_IO_COMP_PWR_DOWN;
 		I915_WRITE(ICL_PHY_MISC(port), val);
 

drivers/gpu/drm/i915/i915_reg.h

@@ -11145,6 +11145,7 @@ enum skl_power_gate {
 #define _ICL_PHY_MISC_B		0x64C04
 #define ICL_PHY_MISC(port)	_MMIO_PORT(port, _ICL_PHY_MISC_A, \
 						 _ICL_PHY_MISC_B)
+#define  ICL_PHY_MISC_MUX_DDID			(1 << 28)
 #define  ICL_PHY_MISC_DE_IO_COMP_PWR_DOWN	(1 << 23)
 
 /* Icelake Display Stream Compression Registers */