When the ATN (Attention) line is asserted during a read we get a
NIUSB_ATN_STATE_ERROR during a read. For the controller to send a
device clear it asserts ATN. Normally this is an error but in the case
of a device clear it should be regarded as an interrupt.

Return -EINTR when the Device Clear Active State (DCAS) is entered
else signal an error with dev_dbg with status instead of just dev_err.

Signed-off-by: Dave Penkler <dpenkler@gmail.com>
---
 drivers/staging/gpib/ni_usb/ni_usb_gpib.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/gpib/ni_usb/ni_usb_gpib.c b/drivers/staging/gpib/ni_usb/ni_usb_gpib.c
index ea44a766fda2..1f8412de9fa3 100644
--- a/drivers/staging/gpib/ni_usb/ni_usb_gpib.c
+++ b/drivers/staging/gpib/ni_usb/ni_usb_gpib.c
@@ -697,8 +697,12 @@ static int ni_usb_read(struct gpib_board *board, u8 *buffer, size_t length,
 		 */
 		break;
 	case NIUSB_ATN_STATE_ERROR:
-		retval = -EIO;
-		dev_err(&usb_dev->dev, "read when ATN set\n");
+		if (status.ibsta & DCAS) {
+			retval = -EINTR;
+		} else {
+			retval = -EIO;
+			dev_dbg(&usb_dev->dev, "read when ATN set stat: 0x%06x\n", status.ibsta);
+		}
 		break;
 	case NIUSB_ADDRESSING_ERROR:
 		retval = -EIO;
-- 
2.51.0