[PATCH v2 1/3] tty: n_gsm: introduce macro for minimal unit size

["D. Starke" <daniel.starke@siemens.com>]

From: Daniel Starke <daniel.starke@siemens.com>

n_gsm has a minimal protocol overhead of 7 bytes. The current code already
checks whether the configured MRU/MTU size is at least one byte more than
this.

Introduce the macro MIN_MTU to make this value more obvious.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

v1 -> v2:
Incorporated review comments.

Link: https://lore.kernel.org/all/fe537e8-7b2-61e2-767d-787b923c7456@linux.intel.com/

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 5e516f5cac5a..570c40a3d78f 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -89,6 +89,7 @@ module_param(debug, int, 0600);
  */
 #define MAX_MRU 1500
 #define MAX_MTU 1500
+#define MIN_MTU (PROT_OVERHEAD + 1)
 /* SOF, ADDR, CTRL, LEN1, LEN2, ..., FCS, EOF */
 #define PROT_OVERHEAD 7
 #define	GSM_NET_TX_TIMEOUT (HZ*10)
@@ -2712,7 +2713,9 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 	if ((c->adaption != 1 && c->adaption != 2) || c->k)
 		return -EOPNOTSUPP;
 	/* Check the MRU/MTU range looks sane */
-	if (c->mru > MAX_MRU || c->mtu > MAX_MTU || c->mru < 8 || c->mtu < 8)
+	if (c->mru < MIN_MTU || c->mtu < MIN_MTU)
+		return -EINVAL;
+	if (c->mru > MAX_MRU || c->mtu > MAX_MTU)
 		return -EINVAL;
 	if (c->n2 > 255)
 		return -EINVAL;
@@ -3296,7 +3299,7 @@ static int gsm_create_network(struct gsm_dlci *dlci, struct gsm_netconfig *nc)
 		return -ENOMEM;
 	}
 	net->mtu = dlci->gsm->mtu;
-	net->min_mtu = 8;
+	net->min_mtu = MIN_MTU;
 	net->max_mtu = dlci->gsm->mtu;
 	mux_net = netdev_priv(net);
 	mux_net->dlci = dlci;
-- 
2.34.1