[PATCH v2 0/1] USB: gadget: f_hid: Add GET_REPORT via userspace IOCTL

[Chris Wulff <Chris.Wulff@xxxxxxxxx>]

This patch has been cleaned up with suggestions from Greg KH
(https://lore.kernel.org/lkml/Y+4Wwm1kFTUEED89@xxxxxxxxx/T/#r12e1bcf3ec45e19d7e2247f7070faac19e0fb60d)

Additional changes since v1:
 * Fixes race conditions during gadget unbind
 * 32/64 bit compatibility fixes (compat ioctl and uapi structure padding)

Additional testing has also been done on Zynq UltraScale+ with the DWC3 driver.

Example userspace code to respond to GET_REPORT
---
#include <stdint.h>
#include <poll.h>
#include <linux/usb/g_hid.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>

void handleGetReport(int hidgfd) {
  uint8_t reportId = 0;
  int ret = ioctl(hidgfd, GADGET_HID_READ_GET_REPORT_ID, &reportId);
  if (ret >= 0 && reportId > 0) {
    struct usb_hidg_report report = {};
    report.report_id = reportId;
    report.userspace_req = 1;
    report.length = 0;

    switch (reportId) {
      case 0x01: // report 0x01 requested
        report.length = 2;
        report.data[0] = reportId, // Report ID
        report.data[1] = 0x00;     // Report Data
        break;

      default:
        // Unknown report (return empty report so the host doesn't have to wait for the timeout)
        break;
    }

    ioctl(hidgfd, GADGET_HID_WRITE_GET_REPORT, &report);
  }
}

int main(int argc, char **argv) {

  int hidgfd = open("/dev/hidg0", O_RDWR | O_NONBLOCK);
  if (hidgfd < 0) {
    return -1;
  }

  struct pollfd fdset[1];

  fdset[0].fd = hidgfd;
  fdset[0].events = POLLIN | POLLPRI;

  int running = 1;
  while (running) {
    int rc, fd;

    fdset[0].revents = 0;

    rc = poll(fdset, 1, 1000); // Wait up to a second

    if (rc > 0) {
      if (fdset[0].revents & POLLPRI) {
        // Process Get Report
        handleGetReport(hidgfd);
      }

      /* POLLIN can be processed here for output reports */
    }
  }

  close(hidgfd);

  return 0;
}