What fields (usages) are required in USB HID reports?

Question

Also see my original question here: Custom HID UPS - the host does not update the data from reports

I'm currently experimenting with developing USB HID devices using the Raspberry Pi Pico microcontroller. Let's consider two cases:

1. A mouse device. Here I've found out that the following report descriptor works on both Windows and Linux:

char ReportDescriptor[48] = {
    0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)
    0x09, 0x02,                    // USAGE (Mouse)
    0xa1, 0x01,                    // COLLECTION (Application)
    0x09, 0x01,                    //   USAGE (Pointer)
    0xa1, 0x00,                    //   COLLECTION (Physical)
    0x05, 0x09,                    //     USAGE_PAGE (Button)
    0x09, 0x01,                    //     USAGE (Button 1)
    0x15, 0x00,                    //     LOGICAL_MINIMUM (0)
    0x25, 0x01,                    //     LOGICAL_MAXIMUM (1)
    0x95, 0x01,                    //     REPORT_COUNT (1)
    0x75, 0x01,                    //     REPORT_SIZE (1)
    0x81, 0x02,                    //     INPUT (Data,Var,Abs)
    0x95, 0x01,                    //     REPORT_COUNT (1)
    0x75, 0x07,                    //     REPORT_SIZE (7)
    0x81, 0x03,                    //     INPUT (Cnst,Var,Abs)
    0x05, 0x01,                    //     USAGE_PAGE (Generic Desktop)
    0x09, 0x30,                    //     USAGE (X)
    0x09, 0x31,                    //     USAGE (Y)
    0x15, 0x80,                    //     LOGICAL_MINIMUM (-128)
    0x25, 0x7f,                    //     LOGICAL_MAXIMUM (127)
    0x95, 0x02,                    //     REPORT_COUNT (2)
    0x75, 0x08,                    //     REPORT_SIZE (8)
    0x81, 0x06,                    //     INPUT (Data,Var,Rel)
    0xc0,                          //   END_COLLECTION
    0xc0                           // END_COLLECTION
};

But the following one doesn't:

char ReportDescriptor[26] = {
    0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)
    0x09, 0x02,                    // USAGE (Mouse)
    0xa1, 0x01,                    // COLLECTION (Application)
    0x09, 0x01,                    //   USAGE (Pointer)
    0xa1, 0x00,                    //   COLLECTION (Physical)
    0x19, 0x30,                    //     USAGE_MINIMUM (X)
    0x29, 0x31,                    //     USAGE_MAXIMUM (Y)
    0x75, 0x08,                    //     REPORT_SIZE (8)
    0x95, 0x02,                    //     REPORT_COUNT (2)
    0x15, 0x80,                    //     LOGICAL_MINIMUM (-128)
    0x25, 0x7f,                    //     LOGICAL_MAXIMUM (127)
    0x81, 0x06,                    //     INPUT (Data,Var,Rel)
    0xc0,                          //   END_COLLECTION
    0xc0                           // END_COLLECTION
};

This leads me to think that operating systems require a mouse to have at least one button.

2. A power device (UPS). The report descriptor looks as follows:

USAGE_PAGE (Power Device)
USAGE (UPS)
COLLECTION (Application)
  USAGE (Battery System)
  COLLECTION (Physical)
    REPORT_ID (1)
    REPORT_SIZE (8)
    REPORT_COUNT (1)

    USAGE (iManufacturer)
    STRING_INDEX (1)
    FEATURE (Constant Variable Absolute)

    USAGE (iProduct)
    STRING_INDEX (2)
    FEATURE (Constant Variable Absolute)

    USAGE (iSerialNumber)
    STRING_INDEX (3)
    FEATURE (Constant Variable Absolute)

    USAGE (Battery)
    COLLECTION (Physical)
      REPORT_ID (2)
      USAGE_PAGE (Battery System)
      REPORT_SIZE (1)
      REPORT_COUNT (4)
      LOGICAL_MINIMUM (0)
      LOGICAL_MAXIMUM (1)
      USAGE (Charging)
      USAGE (Discharging)
      USAGE (Fully Charged)
      USAGE (Fully Discharged)
      INPUT (Variable Absolute Volatile)

However, this one does not work, and I'm struggling to figure out whether it is caused by missing required usages.

So, my questions are as follows:

1. Is the "at least one button in a mouse" requirement set by the operating systems, or the HID spec? Is there any documentation on it?
2. Are there similar requirements for a UPS? Once again, is there any documentation which describes them?

Any information would be appreciated.

Answer 1

Looking at the source code of UPower, it has the following function in src/linux/up-device-hid.c (https://gitlab.freedesktop.org/upower/upower/-/blob/master/src/linux/up-device-hid.c#L253):

static void
up_device_hid_fixup_state (UpDevice *device)
{
    gdouble percentage;

    /* map states the UPS cannot express */
    g_object_get (device, "percentage", &percentage, NULL);
    if (percentage < UP_DAEMON_EPSILON)
        g_object_set (device, "state", UP_DEVICE_STATE_EMPTY, NULL);
    if (percentage > (100.0 - UP_DAEMON_EPSILON))
        g_object_set (device, "state", UP_DEVICE_STATE_FULLY_CHARGED, NULL);
}

So the UPS has to report a RemainingCapacity (RelativeStateOfCharge or AbsoluteStateOfCharge aren't supported for some reason) which should be larger than 0. HID UPSes can actually report empty and fully charged states, so I don't quite understand why UPower is applying this logic unconditionally.

Even though I got the UPS working on Linux, it would still be great to get some information regarding the mouse report descriptor, as well as getting the UPS state recognized on Windows. As such, any amendments to this answer would be appreciated.