使用Libusb 1.0发送中断传输会返回LIBUSB_ERROR_IO，但在接收时不会

Question

I am setting up a testing framework (Google Test Framework) for some custom firmware. I am using USB with the Libusb 1.0 library to receive outputs from the firmware, but I also need to simulate an input to the firmware. I am able to receive the outputs to the host ok, but am not able to send the transfer from the host.

The USB descriptor indicates that interrupt transfers to the host contain 3/4 pieces of data which I capture using this data structure:

typedef struct
{
    uint8_t reportId;
    uint8_t axis[NUM_AXIS];
    uint8_t button;
}usb_report_t;

This works fine using this call from libusb

transfer_error = libusb_interrupt_transfer(dev_handle, 0x81, (unsigned char*)&report, sizeof(report), &transfer_length, 200);

My second task namely sending an interrupt transfer I set up using this structure

typedef struct
{
    uint8_t reportId;
    uint8_t thumper;
}usb_out_report_t;

And I send the interrupt transfer using this call

transfer_error = libusb_interrupt_transfer(dev_handle, 0x01, (unsigned char*)&out_report, sizeof(out_report), &length, 200);

This call returns a -1 which is a LIBUSB_ERROR_IO.

While I was attempting to solve this issue I discovered a work around. If I remove the report ID from the USB descriptor which consequently turns the two structs to

typedef struct
{
    //uint8_t reportId;
    uint8_t axis[NUM_AXIS];
    uint8_t button;
}usb_report_t;
typedef struct
{
    //uint8_t reportId;
    uint8_t thumper;
}usb_out_report_t;

Then the interrupt transfers both in and out work fine. This is ok as a temporary work around but is not an ideal long term solution.

Is there a way to keep the report ID field but have the transfer work both for IN and OUT endpoints?

Answer 1

Had a co-worker help me figure this one out. So apparently in the libusb source code the hid_open() function parses the USB Descriptor and uses the capabilities.NumberOutputValueCaps to determine whether the report ID is needed for writing the output report. The USB Descriptor in my custom firmware gets parsed by this such that the capabilities.NumberOutputValueCaps = 0. This means the api call does not expect a report ID to be used.

To solve this, we just hard coded a particular value for NumberOutputValueCaps and recompiled the libusb library. This let us have a working test framework for the production version of the firmware.