USB: usbtmc: Add USBTMC_IOCTL_GET_STB

This new ioctl reads the status byte (STB) from the device and returns
the STB unmodified to the application. The srq_asserted bit is not taken
into account and not changed.

This ioctl is useful to support non USBTMC-488 compliant devices.
Tested-by: Jian-Wei Wu <jian-wei_wu@keysight.com>
Reviewed-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20201215155621.9592-3-dpenkler@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/usbtmc.c

@@ -2149,6 +2149,12 @@ static long usbtmc_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			file_data->auto_abort = !!tmp_byte;
 		break;
 
+	case USBTMC_IOCTL_GET_STB:
+		retval = usbtmc_get_stb(file_data, &tmp_byte);
+		if (retval > 0)
+			retval = put_user(tmp_byte, (__u8 __user *)arg);
+		break;
+
 	case USBTMC_IOCTL_CANCEL_IO:
 		retval = usbtmc_ioctl_cancel_io(file_data);
 		break;

include/uapi/linux/usb/tmc.h

@@ -102,6 +102,8 @@ struct usbtmc_message {
 #define USBTMC_IOCTL_MSG_IN_ATTR	_IOR(USBTMC_IOC_NR, 24, __u8)
 #define USBTMC_IOCTL_AUTO_ABORT		_IOW(USBTMC_IOC_NR, 25, __u8)
 
+#define USBTMC_IOCTL_GET_STB            _IOR(USBTMC_IOC_NR, 26, __u8)
+
 /* Cancel and cleanup asynchronous calls */
 #define USBTMC_IOCTL_CANCEL_IO		_IO(USBTMC_IOC_NR, 35)
 #define USBTMC_IOCTL_CLEANUP_IO		_IO(USBTMC_IOC_NR, 36)