tty: n_gsm: add helpers to convert mux-num to/from tty-base

Make it obvious how the gsm mux number relates to the virtual tty lines
by using helper functions instead of shifting 6 bits.
Signed-off-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/r/20190710192656.60381-3-martin@geanix.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2171,6 +2171,16 @@ static inline void mux_put(struct gsm_mux *gsm)
 	kref_put(&gsm->ref, gsm_free_muxr);
 }
 
+static inline unsigned int mux_num_to_base(struct gsm_mux *gsm)
+{
+	return gsm->num * NUM_DLCI;
+}
+
+static inline unsigned int mux_line_to_num(unsigned int line)
+{
+	return line / NUM_DLCI;
+}
+
 /**
  *	gsm_alloc_mux		-	allocate a mux
  *
@@ -2351,7 +2361,8 @@ static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
 
 static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 {
-	int ret, i, base;
+	unsigned int base;
+	int ret, i;
 
 	gsm->tty = tty_kref_get(tty);
 	gsm->output = gsmld_output;
@@ -2361,7 +2372,7 @@ static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	else {
 		/* Don't register device 0 - this is the control channel and not
 		   a usable tty interface */
-		base = gsm->num << 6; /* Base for this MUX */
+		base = mux_num_to_base(gsm); /* Base for this MUX */
 		for (i = 1; i < NUM_DLCI; i++)
 			tty_register_device(gsm_tty_driver, base + i, NULL);
 	}
@@ -2379,8 +2390,8 @@ static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 
 static void gsmld_detach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 {
+	unsigned int base = mux_num_to_base(gsm); /* Base for this MUX */
 	int i;
-	int base = gsm->num << 6; /* Base for this MUX */
 
 	WARN_ON(tty != gsm->tty);
 	for (i = 1; i < NUM_DLCI; i++)
@@ -2908,7 +2919,7 @@ static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
 	struct gsm_mux *gsm;
 	struct gsm_dlci *dlci;
 	unsigned int line = tty->index;
-	unsigned int mux = line >> 6;
+	unsigned int mux = mux_line_to_num(line);
 	bool alloc = false;
 	int ret;