[libusb] HID issue on Windows

[Jef D. <jef...@ho...>]

Hi,

I'm having trouble communication with a HID device on Windows. If the WinUSB 
driver is installed, everything works fine. But with the default Microsoft HID 
driver installed, the libusb_interrupt_transfer() function returns 
LIBUSB_ERROR_IO. The underlying WriteFile call fails with 
ERROR_INVALID_PARAMETER. I've attached the debug log file.

When I switch to the hidapi library instead of libusb, then the problem 
disappears. So I checked what hidapi is doing different, and I think I found the 
problem. It seems the Windows HID api always expects to receive a fixed size 
buffer (corresponding to the largest report supported by the device). Therefore 
the hidapi library internally pads the buffer with zeros to the expected size 
when doing a smaller write:

/* Make sure the right number of bytes are passed to WriteFile. Windows
    expects the number of bytes which are in the _longest_ report (plus
    one for the report number) bytes even if the data is a report
    which is shorter than that. Windows gives us this value in
    caps.OutputReportByteLength. If a user passes in fewer bytes than this,
    create a temporary buffer which is the proper size. */
if (length >= dev->output_report_length) {
	/* The user passed the right number of bytes. Use the buffer as-is. */
	buf = (unsigned char *) data;
} else {
	/* Create a temporary buffer and copy the user's data
	   into it, padding the rest with zeros. */
	buf = (unsigned char *) malloc(dev->output_report_length);
	memcpy(buf, data, length);
	memset(buf + length, 0, dev->output_report_length - length);
	length = dev->output_report_length;
}

So I suspect libusb need the same type of fix. The expected size is already 
available in the hid->output_report_size field.

Jef