staging: comedi: COMEDI_BUFINFO: return -EPIPE for abnormal read

The `COMEDI_BUFINFO` ioctl is used to advance the current position in
the buffer by a specified amount (which can be 0) and get the current
position.  If an asynchronous command in the "read" direction has
stopped normally, the command is terminated as soon as the position has
been advanced to the end of all available data.  This is not currently
done if the command terminated with an error.  Change it to allow the
command to be terminated even if it stopped with an error, but report an
`EPIPE` error to the user first.  The `EPIPE` error will not be
reported until the "read" position reported back to the user has been
advanced to the end of all available data.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -686,13 +686,6 @@ static bool __comedi_is_subdevice_running(struct comedi_subdevice *s)
 	return comedi_is_runflags_running(runflags);
 }
 
-static bool comedi_is_subdevice_idle(struct comedi_subdevice *s)
-{
-	unsigned runflags = comedi_get_subdevice_runflags(s);
-
-	return !(runflags & COMEDI_SRF_BUSY_MASK);
-}
-
 bool comedi_can_auto_free_spriv(struct comedi_subdevice *s)
 {
 	unsigned runflags = __comedi_get_subdevice_runflags(s);
@@ -1111,6 +1104,8 @@ static int do_bufinfo_ioctl(struct comedi_device *dev,
 	struct comedi_bufinfo bi;
 	struct comedi_subdevice *s;
 	struct comedi_async *async;
+	unsigned int runflags;
+	int retval = 0;
 	bool become_nonbusy = false;
 
 	if (copy_from_user(&bi, arg, sizeof(bi)))
@@ -1132,9 +1127,20 @@ static int do_bufinfo_ioctl(struct comedi_device *dev,
 			comedi_buf_read_alloc(s, bi.bytes_read);
 			bi.bytes_read = comedi_buf_read_free(s, bi.bytes_read);
 		}
-		if (comedi_is_subdevice_idle(s) &&
-		    comedi_buf_read_n_available(s) == 0)
+		/*
+		 * If nothing left to read, and command has stopped, and
+		 * {"read" position not updated or command stopped normally},
+		 * then become non-busy.
+		 */
+		runflags = comedi_get_subdevice_runflags(s);
+		if (comedi_buf_read_n_available(s) == 0 &&
+		    !comedi_is_runflags_running(runflags) &&
+		    (bi.bytes_read == 0 ||
+		     !comedi_is_runflags_in_error(runflags))) {
 			become_nonbusy = true;
+			if (comedi_is_runflags_in_error(runflags))
+				retval = -EPIPE;
+		}
 		bi.bytes_written = 0;
 	} else {
 		/* command was set up in "write" direction */
@@ -1156,6 +1162,9 @@ static int do_bufinfo_ioctl(struct comedi_device *dev,
 	if (become_nonbusy)
 		do_become_nonbusy(dev, s);
 
+	if (retval)
+		return retval;
+
 	if (copy_to_user(arg, &bi, sizeof(bi)))
 		return -EFAULT;