On Sat, 27 Feb 2021 17:26:42 -0800
Ronald Tschalär [off-list ref] wrote:

The iBridge device provides access to several devices, including:
- the Touch Bar
- the iSight webcam
- the light sensor
- the fingerprint sensor

This driver provides the core support for managing the iBridge device
and the access to the underlying devices. In particular, the
functionality for the touch bar and light sensor is exposed via USB HID
interfaces, and on devices with the T1 chip one of the HID devices is
used for both functions. So this driver creates virtual HID devices, one
per top-level report collection on each HID device (for a total of 3
virtual HID devices). The sub-drivers then bind to these virtual HID
devices.

This way the Touch Bar and ALS drivers can be kept in their own modules,
while at the same time making them look very much like as if they were
connected to the real HID devices. And those drivers then work (mostly)
without further changes on MacBooks with the T2 chip that don't need
this driver.

Signed-off-by: Ronald Tschalär <redacted>

Hi Ronald.

This is far from my specialty but mostly looks sensible to me.

Just one thing stood out that I couldn't understand. See below.

Jonathan

...

+
+#ifdef CONFIG_PM
+/**
+ * appleib_forward_int_op() - Forward a hid-driver callback to all drivers on
+ * all virtual HID devices attached to the given real HID device.
+ * @hdev the real hid-device
+ * @forward a function that calls the callback on the given driver
+ * @args arguments for the forward function
+ *
+ * This is for callbacks that return a status as an int.
+ *
+ * Returns: 0 on success, or the first error returned by the @forward function.
+ */
+static int appleib_forward_int_op(struct hid_device *hdev,
+				  int (*forward)(struct hid_driver *,
+						 struct hid_device *, void *),
+				  void *args)
+{
+	struct appleib_hid_dev_info *hdev_info = hid_get_drvdata(hdev);
+	struct hid_device *sub_hdev;
+	int rc = 0;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(hdev_info->sub_hdevs); i++) {
+		sub_hdev = hdev_info->sub_hdevs[i];
+		if (sub_hdev->driver) {
+			rc = forward(sub_hdev->driver, sub_hdev, args);
+			if (rc)
+				break;

return rc; here would be cleaner.

+		}
+
+		break;

This is unusual.  It's a for loop but as far as I can see only first iteration
can ever run as we exit the loop at this break if we haven't done so earlier.
What is the intent here?

+	}
+
+	return rc;
+}