thinkpad-acpi: untangle ACPI/vendor backlight selection

acpi_video_backlight_support() already tells us if ACPI is handling
backlight control through the generic ACPI handle.  It is better to just
trust it.

While at it, adjust down a printk priority, and test earlier for
brightness_enable=0.
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

Documentation/laptops/thinkpad-acpi.txt

@@ -1024,6 +1024,10 @@ ThinkPad-specific interface.  The driver will disable its native
 backlight brightness control interface if it detects that the standard
 ACPI interface is available in the ThinkPad.
 
+If you want to use the thinkpad-acpi backlight brightness control
+instead of the generic ACPI video backlight brightness control for some
+reason, you should use the acpi_backlight=vendor kernel parameter.
+
 The brightness_enable module parameter can be used to control whether
 the LCD brightness control feature will be enabled when available.
 brightness_enable=0 forces it to be disabled.  brightness_enable=1

drivers/platform/x86/thinkpad_acpi.c

@@ -6216,34 +6216,32 @@ static int __init brightness_init(struct ibm_init_struct *iibm)
 	if (tp_features.bright_unkfw)
 		return 1;
 
-	if (tp_features.bright_acpimode) {
+	if (!brightness_enable) {
+		dbg_printk(TPACPI_DBG_INIT | TPACPI_DBG_BRGHT,
+			   "brightness support disabled by "
+			   "module parameter\n");
+		return 1;
+	}
+
 	if (acpi_video_backlight_support()) {
 		if (brightness_enable > 1) {
-				printk(TPACPI_NOTICE
+			printk(TPACPI_INFO
 			       "Standard ACPI backlight interface "
 			       "available, not loading native one.\n");
 			return 1;
 		} else if (brightness_enable == 1) {
-				printk(TPACPI_NOTICE
-				       "Backlight control force enabled, even if standard "
-				       "ACPI backlight interface is available\n");
+			printk(TPACPI_WARN
+				"Cannot enable backlight brightness support, "
+				"ACPI is already handling it.  Refer to the "
+				"acpi_backlight kernel parameter\n");
+			return 1;
 		}
-		} else {
-			if (brightness_enable > 1) {
+	} else if (tp_features.bright_acpimode && brightness_enable > 1) {
 		printk(TPACPI_NOTICE
 			"Standard ACPI backlight interface not "
 			"available, thinkpad_acpi native "
 			"brightness control enabled\n");
 	}
-		}
-	}
-
-	if (!brightness_enable) {
-		dbg_printk(TPACPI_DBG_INIT | TPACPI_DBG_BRGHT,
-			   "brightness support disabled by "
-			   "module parameter\n");
-		return 1;
-	}
 
 	/*
 	 * Check for module parameter bogosity, note that we