PCI / pci-label: treat PCI label with index 0 as valid label

Current pci-label driver detects ACPI label by checking label index
returned by ACPI _DSM method, and treats it as valid if label index
is positive. According to ACPI Firmware specification 3.1, zero is
also an valid label index. So change code to detect availability of
ACPI slot label by checking availaiblity of ACPI _DSM function for
PCI label.
Signed-off-by: NJiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pci/pci-label.c

@@ -187,7 +187,6 @@ static const char device_label_dsm_uuid[] = {
 };
 
 enum acpi_attr_enum {
-	ACPI_ATTR_NONE = 0,
 	ACPI_ATTR_LABEL_SHOW,
 	ACPI_ATTR_INDEX_SHOW,
 };
@@ -222,20 +221,16 @@ dsm_get_label(struct device *dev, char *buf, enum acpi_attr_enum attr)
 	if (obj->type == ACPI_TYPE_PACKAGE && obj->package.count == 2 &&
 	    tmp[0].type == ACPI_TYPE_INTEGER &&
 	    tmp[1].type == ACPI_TYPE_STRING) {
-		len = tmp[0].integer.value;
-		if (buf) {
-			/*
-			 * This second string element is optional even when
-			 * this _DSM is implemented; when not implemented,
-			 * this entry must return a null string.
-			 */
-			if (attr == ACPI_ATTR_INDEX_SHOW)
-				scnprintf(buf, PAGE_SIZE, "%llu\n",
-				tmp->integer.value);
-			else if (attr == ACPI_ATTR_LABEL_SHOW)
-				dsm_label_utf16s_to_utf8s(tmp + 1, buf);
-			len = strlen(buf) > 0 ? strlen(buf) : -1;
-		}
+		/*
+		 * The second string element is optional even when
+		 * this _DSM is implemented; when not implemented,
+		 * this entry must return a null string.
+		 */
+		if (attr == ACPI_ATTR_INDEX_SHOW)
+			scnprintf(buf, PAGE_SIZE, "%llu\n", tmp->integer.value);
+		else if (attr == ACPI_ATTR_LABEL_SHOW)
+			dsm_label_utf16s_to_utf8s(tmp + 1, buf);
+		len = strlen(buf) > 0 ? strlen(buf) : -1;
 	}
 
 	ACPI_FREE(obj);
@@ -246,7 +241,14 @@ dsm_get_label(struct device *dev, char *buf, enum acpi_attr_enum attr)
 static bool
 device_has_dsm(struct device *dev)
 {
-	return dsm_get_label(dev, NULL, ACPI_ATTR_NONE) > 0;
+	acpi_handle handle;
+
+	handle = ACPI_HANDLE(dev);
+	if (!handle)
+		return false;
+
+	return !!acpi_check_dsm(handle, device_label_dsm_uuid, 0x2,
+				1 << DEVICE_LABEL_DSM);
 }
 
 static umode_t