diff --git a/drivers/staging/comedi/comedi_buf.c b/drivers/staging/comedi/comedi_buf.c
index 220816857128..ac498e0b4865 100644
--- a/drivers/staging/comedi/comedi_buf.c
+++ b/drivers/staging/comedi/comedi_buf.c
@@ -323,8 +323,10 @@ static unsigned int comedi_buf_munge(struct comedi_async *async,
 	return count;
 }
 
-unsigned int comedi_buf_write_n_allocated(struct comedi_async *async)
+unsigned int comedi_buf_write_n_allocated(struct comedi_subdevice *s)
 {
+	struct comedi_async *async = s->async;
+
 	return async->buf_write_alloc_count - async->buf_write_count;
 }
 
@@ -333,7 +335,7 @@ unsigned int comedi_buf_write_free(struct comedi_subdevice *s,
 				   unsigned int nbytes)
 {
 	struct comedi_async *async = s->async;
-	unsigned int allocated = comedi_buf_write_n_allocated(async);
+	unsigned int allocated = comedi_buf_write_n_allocated(s);
 
 	if (nbytes > allocated)
 		nbytes = allocated;
diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index c7010b315701..bc9386350b76 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2040,7 +2040,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 		poll_wait(file, &s->async->wait_head, wait);
 		comedi_buf_write_alloc(s, s->async->prealloc_bufsz);
 		if (!s->busy || !comedi_is_subdevice_running(s) ||
-		    comedi_buf_write_n_allocated(s->async) >= bps)
+		    comedi_buf_write_n_allocated(s) >= bps)
 			mask |= POLLOUT | POLLWRNORM;
 	}
 
@@ -2137,8 +2137,8 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 		if (async->buf_write_ptr + m > async->prealloc_bufsz)
 			m = async->prealloc_bufsz - async->buf_write_ptr;
 		comedi_buf_write_alloc(s, async->prealloc_bufsz);
-		if (m > comedi_buf_write_n_allocated(async))
-			m = comedi_buf_write_n_allocated(async);
+		if (m > comedi_buf_write_n_allocated(s))
+			m = comedi_buf_write_n_allocated(s);
 		if (m < n)
 			n = m;
 
diff --git a/drivers/staging/comedi/comedi_internal.h b/drivers/staging/comedi/comedi_internal.h
index a492f2d2436e..36e7a99b5079 100644
--- a/drivers/staging/comedi/comedi_internal.h
+++ b/drivers/staging/comedi/comedi_internal.h
@@ -21,7 +21,7 @@ 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_allocated(struct comedi_async *async);
+unsigned int comedi_buf_write_n_allocated(struct comedi_subdevice *s);
 void comedi_device_cancel_all(struct comedi_device *dev);
 
 extern unsigned int comedi_default_buf_size_kb;