usb_hid: honor a boot protocol request that arrives after startup

shared-bindings/usb_hid/Device.c

@@ -156,6 +156,10 @@ static mp_obj_t usb_hid_device_make_new(const mp_obj_type_t *type, size_t n_args
 //|         The ``report`` itself will be discarded, to prevent unwanted extraneous characters,
 //|         mouse clicks, etc.
 //|
+//|         If the host has put the interface in boot protocol, the boot device sends its report
+//|         without a report ID, and `send_report()` on any other device discards the report,
+//|         because the host reads only the boot device.
+//|
 //|         Note: Host operating systems allow enabling and disabling specific devices
 //|         and kinds of devices to do wakeup.
 //|         The defaults are different for different operating systems.

shared-bindings/usb_hid/__init__.c

@@ -25,6 +25,10 @@
 //| the `devices` tuple is **replaced** when ``code.py`` starts with a single-element tuple
 //| containing a `Device` that describes the boot device chosen (keyboard or mouse).
 //| The request for a boot device overrides any other HID devices.
+//|
+//| If the host requests a boot device after ``code.py`` has started, `devices` is not replaced.
+//| The boot device then sends reports in the boot format, with no report ID, and
+//| `Device.send_report()` on any other device does nothing, because the host ignores it.
 //| """
 //|
 //|

shared-module/usb_hid/Device.c

@@ -130,6 +130,19 @@ void common_hal_usb_hid_device_send_report(usb_hid_device_obj_t *self, uint8_t *
             mp_raise_msg(&mp_type_OSError, MP_ERROR_TEXT("USB busy"));
         }
 
+        const uint8_t boot_device = usb_hid_boot_device();
+        if (boot_device != 0 && tud_hid_get_protocol() == HID_PROTOCOL_BOOT) {
+            // In boot protocol the host reads the fixed boot report, which carries no report ID,
+            // and ignores every device other than the boot device.
+            if (self->usage_page == HID_USAGE_PAGE_DESKTOP &&
+                ((boot_device == 1 && self->usage == HID_USAGE_DESKTOP_KEYBOARD) ||
+                 (boot_device == 2 && self->usage == HID_USAGE_DESKTOP_MOUSE))) {
+                report_id = 0;
+            } else {
+                return;
+            }
+        }
+
         if (!tud_hid_report(report_id, report, len)) {
             mp_raise_msg(&mp_type_OSError, MP_ERROR_TEXT("USB error"));
         }