HID: core: add helper for finding a field with a certain usage

This helper will allow HID drivers to easily determine if they should
bind to a hid_device by checking for the prescence of a certain field
when its ID is not enough, which can be the case on USB devices with
multiple interfaces and/or configurations.

Convert google-hammer driver to use it, and remove now superfluous
hammer_has_usage().

[jkosina@suse.com: expand changelog with the information about
 google-hammer being added as user of this API ]
Signed-off-by: Kerem Karabay <kekrby@gmail.com>
Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

drivers/hid/hid-core.c

@@ -1912,6 +1912,31 @@ int hid_set_field(struct hid_field *field, unsigned offset, __s32 value)
 }
 EXPORT_SYMBOL_GPL(hid_set_field);
 
+struct hid_field *hid_find_field(struct hid_device *hdev, unsigned int report_type,
+				 unsigned int application, unsigned int usage)
+{
+	struct list_head *report_list = &hdev->report_enum[report_type].report_list;
+	struct hid_report *report;
+	int i, j;
+
+	list_for_each_entry(report, report_list, list) {
+		if (report->application != application)
+			continue;
+
+		for (i = 0; i < report->maxfield; i++) {
+			struct hid_field *field = report->field[i];
+
+			for (j = 0; j < field->maxusage; j++) {
+				if (field->usage[j].hid == usage)
+					return field;
+			}
+		}
+	}
+
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(hid_find_field);
+
 static struct hid_report *hid_get_report(struct hid_report_enum *report_enum,
 		const u8 *data)
 {

drivers/hid/hid-google-hammer.c

@@ -418,38 +418,15 @@ static int hammer_event(struct hid_device *hid, struct hid_field *field,
 	return 0;
 }
 
-static bool hammer_has_usage(struct hid_device *hdev, unsigned int report_type,
-			unsigned application, unsigned usage)
-{
-	struct hid_report_enum *re = &hdev->report_enum[report_type];
-	struct hid_report *report;
-	int i, j;
-
-	list_for_each_entry(report, &re->report_list, list) {
-		if (report->application != application)
-			continue;
-
-		for (i = 0; i < report->maxfield; i++) {
-			struct hid_field *field = report->field[i];
-
-			for (j = 0; j < field->maxusage; j++)
-				if (field->usage[j].hid == usage)
-					return true;
-		}
-	}
-
-	return false;
-}
-
 static bool hammer_has_folded_event(struct hid_device *hdev)
 {
-	return hammer_has_usage(hdev, HID_INPUT_REPORT,
+	return !!hid_find_field(hdev, HID_INPUT_REPORT,
 				HID_GD_KEYBOARD, HID_USAGE_KBD_FOLDED);
 }
 
 static bool hammer_has_backlight_control(struct hid_device *hdev)
 {
-	return hammer_has_usage(hdev, HID_OUTPUT_REPORT,
+	return !!hid_find_field(hdev, HID_OUTPUT_REPORT,
 				HID_GD_KEYBOARD, HID_AD_BRIGHTNESS);
 }
 

include/linux/hid.h

@@ -940,6 +940,8 @@ extern void hidinput_report_event(struct hid_device *hid, struct hid_report *rep
 extern int hidinput_connect(struct hid_device *hid, unsigned int force);
 extern void hidinput_disconnect(struct hid_device *);
 
+struct hid_field *hid_find_field(struct hid_device *hdev, unsigned int report_type,
+				 unsigned int application, unsigned int usage);
 int hid_set_field(struct hid_field *, unsigned, __s32);
 int hid_input_report(struct hid_device *hid, enum hid_report_type type, u8 *data, u32 size,
 		     int interrupt);