Windows USB HID Report Length

Question

I am developing a USB HID device using an STMicro microcontroller. I started with STMicro's HID example which works fine. I am using C++ on Windows 7 64-bit for the PC side. I have an application that works with my device. There is one thing I can't figure out, however.

The example firmware only allowed sending and receiving 2 bytes at a time, which is determined by a HIDP_CAPS.OutputReportByteLength and InputReportByteLength. I would like to send more data than this at once, but I can't figure out how to increase the report lengths. I successfully changed the endpoint wMaxPacketSize, the VID and PID, and a few other things, but I can't figure out how Windows is calculating the in and out report lengths. There doesn't seem to be any fields in my report or device descriptions that indicate this length, but I can't imagine where else it might be coming from.

Can anyone tell me how Windows determines the HIDP_CAPS.OutputReportByteLength and HIDP_CAPS.InputReportByteLength?

How can I increase these lengths?

Answer 1

I figured it out. I thought I would post here in case anyone else needs to know. I'm not entirely sure I really understand it all, so if I made a mistake, someone please correct me.

I had to change the report description in my firmware. I had several usages. Windows gets the report description and figures out which usage requires the longest length and uses that length. On one of my input reports I made the following changes (the input report is just an array of bytes in firmware):

0x27, 0xFF, 0xFF, 0xFF, 0xFF, //Logical maximum is 4 bytes long, and has a value of 0xFFFFFFFF 0x95, 0x01, //There is one report 0x75, 0x20, //There are 32 bits per report

I did something similar for the output, but there is no report number field (0x95).

Windows now tells me I can send and receive 5 bytes, which I believe means the end point plus report number times report size.