Posted on November 05, 2016 at 14:06Hi rapid84,
On high-speed (HS) bus also, the MPS (Max Packet Size) of HID interrupt endpoint(s) is limited to 64 bytes, under current implementation of built-in HID driver on major OS (Windows, Mac and Linux).
By the USB 2.0 spec, HS interrupt endpoint may have MPS of greater than 64 bytes. But such an endpoint should be declared on an alternate interface, not on the default one. Unfortunately, current OS HID drivers don't have any API to switch to the alternate interface. And then, above limitation is applied.
Even under above limitation, you may declare ''report size'' of greater than 64 bytes on the report descriptor.
Such large reports are exchanged by splitting into 64 bytes packets on the bus. USBD_HID_SendReport() accepts such a large report in single call. The stack and the device hardware split the report into packets of MPS. On the host side, host controller hardware gathers the packets into single report. Your HID host application retrieves full-size report in single API call. In this way, you don't need to be aware of the MPS at all, except that each packet takes (micro-)frame to be carried over. (If the transfer speed would be in your concern for such a large-size report, apply Set_/Get_Report over the default endpoint, which should be faster than interrupt endpoints)
Tsuneo
