staging: comedi: add new comedi_buf_write_n_available()

Add a new function `comedi_buf_write_n_available()` to return the amount
of buffer space available for writing, including space already allocated
by `comedi_buf_write_alloc()` plus any unallocated space available.
This is currently just for internal use by the comedi core, so is not
exported.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_buf.c

@@ -253,6 +253,14 @@ static unsigned int comedi_buf_write_n_unalloc(struct comedi_subdevice *s)
 	return free_end - async->buf_write_alloc_count;
 }
 
+unsigned int comedi_buf_write_n_available(struct comedi_subdevice *s)
+{
+	struct comedi_async *async = s->async;
+	unsigned int free_end = async->buf_read_count + async->prealloc_bufsz;
+
+	return free_end - async->buf_write_count;
+}
+
 /**
  * comedi_buf_write_alloc() - Reserve buffer space for writing
  * @s: COMEDI subdevice.

drivers/staging/comedi/comedi_internal.h

@@ -31,6 +31,7 @@ void comedi_buf_map_get(struct comedi_buf_map *bm);
 int comedi_buf_map_put(struct comedi_buf_map *bm);
 struct comedi_buf_map *comedi_buf_map_from_subdev_get(
 		struct comedi_subdevice *s);
+unsigned int comedi_buf_write_n_available(struct comedi_subdevice *s);
 unsigned int comedi_buf_write_n_allocated(struct comedi_subdevice *s);
 void comedi_device_cancel_all(struct comedi_device *dev);
 bool comedi_can_auto_free_spriv(struct comedi_subdevice *s);