drm/i915/pch: abstract fake PCH detection better

Abstract detection of platforms with south display on the same PCI
device or SoC die as north display, and all around clarify what this
means. Debug log about it for good measure.

Reviewed-by: Chaitanya Kumar Borah <[e-mail]>
Link: https://lore.kernel.org/r/95c[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

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

···
 #define INTEL_PCH_P3X_DEVICE_ID_TYPE		0x7000
 #define INTEL_PCH_QEMU_DEVICE_ID_TYPE		0x2900 /* qemu q35 has 2918 */
 
+/*
+ * Check for platforms where the south display is on the same PCI device or SoC
+ * die as the north display. The PCH (if it even exists) is not involved in
+ * display. Return a fake PCH type for south display handling on these
+ * platforms, without actually detecting the PCH, and PCH_NONE otherwise.
+ */
+static enum intel_pch intel_pch_fake_for_south_display(struct intel_display *display)
+{
+	enum intel_pch pch_type = PCH_NONE;
+
+	if (DISPLAY_VER(display) >= 20)
+		pch_type = PCH_LNL;
+	else if (display->platform.battlemage || display->platform.meteorlake)
+		pch_type = PCH_MTL;
+	else if (display->platform.dg2)
+		pch_type = PCH_DG2;
+	else if (display->platform.dg1)
+		pch_type = PCH_DG1;
+
+	return pch_type;
+}
+
 /* Map PCH device id to PCH type, or PCH_NONE if unknown. */
 static enum intel_pch
 intel_pch_type(const struct intel_display *display, unsigned short id)
···
 	unsigned short id;
 	enum intel_pch pch_type;
 
-	/*
-	 * South display engine on the same PCI device: just assign the fake
-	 * PCH.
-	 */
-	if (DISPLAY_VER(display) >= 20) {
-		display->pch_type = PCH_LNL;
-		return;
-	} else if (display->platform.battlemage || display->platform.meteorlake) {
-		/*
-		 * Both north display and south display are on the SoC die.
-		 * The real PCH (if it even exists) is uninvolved in display.
-		 */
-		display->pch_type = PCH_MTL;
-		return;
-	} else if (display->platform.dg2) {
-		display->pch_type = PCH_DG2;
-		return;
-	} else if (display->platform.dg1) {
-		display->pch_type = PCH_DG1;
+	pch_type = intel_pch_fake_for_south_display(display);
+	if (pch_type != PCH_NONE) {
+		display->pch_type = pch_type;
+		drm_dbg_kms(display->drm,
+			    "PCH not involved in display, using fake PCH type %d for south display\n",
+			    pch_type);
 		return;
 	}