n_tty: Use DIV_ROUND_UP() in room calculation

When PARMRK is set, a character can result in up to 3 chars in the read
buffer. Receive code calculates for how many characters there (at
least) is room. Convert an opencoded rounding in the calculation to use
DIV_ROUND_UP().

Note: the room variable is decremented afterwards by one which ensures
the characters will fit into the buffer for real so the code is okay
despite rounding upwards.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230309082035.14880-6-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 2cf263de1366b8185d90b00025dc1f46c7c4216e..6d754fc35dcebd5c3ab1e44f8989230fae2d8e60 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -36,6 +36,7 @@
 #include <linux/file.h>
 #include <linux/interrupt.h>
 #include <linux/major.h>
+#include <linux/math.h>
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/poll.h>
@@ -1692,7 +1693,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 
                room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
                if (I_PARMRK(tty))
-                       room = (room + 2) / 3;
+                       room = DIV_ROUND_UP(room, 3);
                room--;
                if (room <= 0) {
                        overflow = ldata->icanon && ldata->canon_head == tail;