staging: comedi: drivers: introduce comedi_nscans_left()

Introduce a helper function to determine the number of scans left in
the async command.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedidev.h

@@ -516,6 +516,8 @@ int comedi_dio_insn_config(struct comedi_device *, struct comedi_subdevice *,
 unsigned int comedi_dio_update_state(struct comedi_subdevice *,
 				     unsigned int *data);
 unsigned int comedi_bytes_per_scan(struct comedi_subdevice *s);
+unsigned int comedi_nscans_left(struct comedi_subdevice *s,
+				unsigned int nscans);
 void comedi_inc_scan_progress(struct comedi_subdevice *s,
 			      unsigned int num_bytes);
 

drivers/staging/comedi/drivers.c

@@ -327,6 +327,45 @@ unsigned int comedi_bytes_per_scan(struct comedi_subdevice *s)
 }
 EXPORT_SYMBOL_GPL(comedi_bytes_per_scan);
 
+/**
+ * comedi_nscans_left - return the number of scans left in the command
+ * @s: comedi_subdevice struct
+ * @nscans: the expected number of scans
+ *
+ * If nscans is 0, the number of scans available in the async buffer will be
+ * used. Otherwise the expected number of scans will be used.
+ *
+ * If the async command has a stop_src of TRIG_COUNT, the nscans will be
+ * checked against the number of scans left in the command.
+ *
+ * The return value will then be either the expected number of scans or the
+ * number of scans remaining in the command.
+ */
+unsigned int comedi_nscans_left(struct comedi_subdevice *s,
+				unsigned int nscans)
+{
+	struct comedi_async *async = s->async;
+	struct comedi_cmd *cmd = &async->cmd;
+
+	if (nscans == 0) {
+		unsigned int nbytes = comedi_buf_read_n_available(s);
+
+		nscans = nbytes / comedi_bytes_per_scan(s);
+	}
+
+	if (cmd->stop_src == TRIG_COUNT) {
+		unsigned int scans_left = 0;
+
+		if (async->scans_done < cmd->stop_arg)
+			scans_left = cmd->stop_arg - async->scans_done;
+
+		if (nscans > scans_left)
+			nscans = scans_left;
+	}
+	return nscans;
+}
+EXPORT_SYMBOL_GPL(comedi_nscans_left);
+
 /**
  * comedi_inc_scan_progress - update scan progress in asynchronous command
  * @s: comedi_subdevice struct