USB: usbtmc: Add separate USBTMC_IOCTL_GET_SRQ_STB

This new ioctl only returns the status byte (STB) that was originally
sent by the device due to a service request (SRQ) condition.

This ioctl checks the srq_asserted bit of the associated file
descriptor. If set, the srq_asserted bit is reset and the cached
STB with original SRQ information is returned. Otherwise the ioctl
returns the error code ENOMSG.

This ioctl is useful to support non USBTMC-488 compliant devices.
Time sensitive applications can read the cached STB without incurring
the cost of an urb transaction over the bus.

Tested-by: Jian-Wei Wu <[e-mail]>
Reviewed-by: Guido Kiener <[e-mail]>
Signed-off-by: Dave Penkler <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/usbtmc.c

···
 
 }
 
+static int usbtmc_ioctl_get_srq_stb(struct usbtmc_file_data *file_data,
+				void __user *arg)
+{
+	struct usbtmc_device_data *data = file_data->data;
+	struct device *dev = &data->intf->dev;
+	int srq_asserted = 0;
+	__u8 stb = 0;
+	int rv;
+
+	spin_lock_irq(&data->dev_lock);
+	srq_asserted  = atomic_xchg(&file_data->srq_asserted, srq_asserted);
+
+	if (srq_asserted) {
+		stb = file_data->srq_byte;
+		spin_unlock_irq(&data->dev_lock);
+		rv = put_user(stb, (__u8 __user *)arg);
+	} else {
+		spin_unlock_irq(&data->dev_lock);
+		rv = -ENOMSG;
+	}
+
+	dev_dbg(dev, "stb:0x%02x with srq received %d\n", (unsigned int)stb, rv);
+
+	return rv;
+}
+
 static int usbtmc488_ioctl_wait_srq(struct usbtmc_file_data *file_data,
 				    __u32 __user *arg)
 {
···
 		retval = usbtmc_get_stb(file_data, &tmp_byte);
 		if (retval > 0)
 			retval = put_user(tmp_byte, (__u8 __user *)arg);
+		break;
+
+	case USBTMC_IOCTL_GET_SRQ_STB:
+		retval = usbtmc_ioctl_get_srq_stb(file_data,
+						  (void __user *)arg);
 		break;
 
 	case USBTMC_IOCTL_CANCEL_IO:

include/uapi/linux/usb/tmc.h

···
 #define USBTMC_IOCTL_AUTO_ABORT		_IOW(USBTMC_IOC_NR, 25, __u8)
 
 #define USBTMC_IOCTL_GET_STB            _IOR(USBTMC_IOC_NR, 26, __u8)
+#define USBTMC_IOCTL_GET_SRQ_STB        _IOR(USBTMC_IOC_NR, 27, __u8)
 
 /* Cancel and cleanup asynchronous calls */
 #define USBTMC_IOCTL_CANCEL_IO		_IO(USBTMC_IOC_NR, 35)