HID: Scan the device for group info before adding it

In order to allow the report descriptor to influence the hid device
properties, one needs to parse the descriptor early, without reference
to any driver. Scan the descriptor for group information during device
add, before the device has been broadcast to userland. The device
modalias will contain group information which can be used to
differentiate between modules. For starters, just handle the generic
group.

Signed-off-by: Henrik Rydberg <[e-mail]>
Acked-by: Benjamin Tissoires <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/hid-core.c

···
 	return NULL;
 }
 
+static void hid_scan_usage(struct hid_device *hid, u32 usage)
+{
+}
+
+/*
+ * Scan a report descriptor before the device is added to the bus.
+ * Sets device groups and other properties that determine what driver
+ * to load.
+ */
+static int hid_scan_report(struct hid_device *hid)
+{
+	unsigned int page = 0, delim = 0;
+	__u8 *start = hid->dev_rdesc;
+	__u8 *end = start + hid->dev_rsize;
+	unsigned int u, u_min = 0, u_max = 0;
+	struct hid_item item;
+
+	hid->group = HID_GROUP_GENERIC;
+	while ((start = fetch_item(start, end, &item)) != NULL) {
+		if (item.format != HID_ITEM_FORMAT_SHORT)
+			return -EINVAL;
+		if (item.type == HID_ITEM_TYPE_GLOBAL) {
+			if (item.tag == HID_GLOBAL_ITEM_TAG_USAGE_PAGE)
+				page = item_udata(&item) << 16;
+		} else if (item.type == HID_ITEM_TYPE_LOCAL) {
+			if (delim > 1)
+				break;
+			u = item_udata(&item);
+			if (item.size <= 2)
+				u += page;
+			switch (item.tag) {
+			case HID_LOCAL_ITEM_TAG_DELIMITER:
+				delim += !!u;
+				break;
+			case HID_LOCAL_ITEM_TAG_USAGE:
+				hid_scan_usage(hid, u);
+				break;
+			case HID_LOCAL_ITEM_TAG_USAGE_MINIMUM:
+				u_min = u;
+				break;
+			case HID_LOCAL_ITEM_TAG_USAGE_MAXIMUM:
+				u_max = u;
+				for (u = u_min; u <= u_max; u++)
+					hid_scan_usage(hid, u);
+				break;
+			}
+		}
+	}
+
+	return 0;
+}
+
 /**
  * hid_parse_report - parse device report
  *
···
 		return ret;
 	if (!hdev->dev_rdesc)
 		return -ENODEV;
+
+	/*
+	 * Scan generic devices for group information
+	 */
+	if (hid_ignore_special_drivers ||
+	    !hid_match_id(hdev, hid_have_special_driver)) {
+		ret = hid_scan_report(hdev);
+		if (ret)
+			hid_warn(hdev, "bad device descriptor (%d)\n", ret);
+	}
 
 	/* XXX hack, any other cleaner solution after the driver core
 	 * is converted to allow more than 20 bytes as the device name? */

include/linux/hid.h

···
 #define HID_QUIRK_NO_INPUT_SYNC			0x80000000
 
 /*
+ * HID device groups
+ */
+#define HID_GROUP_GENERIC			0x0001
+
+/*
  * This is the global environment of the parser. This information is
  * persistent for main-items. The global environment can be saved and
  * restored with PUSH/POP statements.