--- /dev/null
+From 19317433057dc1f2ca9a975e4e6b547282c2a5ef Mon Sep 17 00:00:00 2001
+From: Daniel Starke <daniel.starke@siemens.com>
+Date: Mon, 25 Apr 2022 03:47:26 -0700
+Subject: tty: n_gsm: fix sometimes uninitialized warning in gsm_dlci_modem_output()
+
+From: Daniel Starke <daniel.starke@siemens.com>
+
+commit 19317433057dc1f2ca9a975e4e6b547282c2a5ef upstream.
+
+'size' may be used uninitialized in gsm_dlci_modem_output() if called with
+an adaption that is neither 1 nor 2. The function is currently only called
+by gsm_modem_upd_via_data() and only for adaption 2.
+Properly handle every invalid case by returning -EINVAL to silence the
+compiler warning and avoid future regressions.
+
+Fixes: c19ffe00fed6 ("tty: n_gsm: fix invalid use of MSC in advanced option")
+Cc: stable@vger.kernel.org
+Reported-by: kernel test robot <lkp@intel.com>
+Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
+Link: https://lore.kernel.org/r/20220425104726.7986-1-daniel.starke@siemens.com
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ drivers/tty/n_gsm.c | 15 +++++++++------
+ 1 file changed, 9 insertions(+), 6 deletions(-)
+
+--- a/drivers/tty/n_gsm.c
++++ b/drivers/tty/n_gsm.c
+@@ -932,18 +932,21 @@ static int gsm_dlci_modem_output(struct
+ {
+ u8 *dp = NULL;
+ struct gsm_msg *msg;
+- int size;
++ int size = 0;
+
+ /* for modem bits without break data */
+- if (dlci->adaption == 1) {
+- size = 0;
+- } else if (dlci->adaption == 2) {
+- size = 1;
++ switch (dlci->adaption) {
++ case 1: /* Unstructured */
++ break;
++ case 2: /* Unstructured with modem bits. */
++ size++;
+ if (brk > 0)
+ size++;
+- } else {
++ break;
++ default:
+ pr_err("%s: unsupported adaption %d\n", __func__,
+ dlci->adaption);
++ return -EINVAL;
+ }
+
+ msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);