Date:   Wed, 12 Oct 2022 08:17:15 +0200
From:   "D. Starke" <daniel.starke@...mens.com>
To:     linux-serial@...r.kernel.org, gregkh@...uxfoundation.org,
        jirislaby@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Daniel Starke <daniel.starke@...mens.com>
Subject: [PATCH 1/1] tty: n_gsm: make n_gsm line number configurable

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

Currently, the n_gsm line number and its derived virtual ttys are assigned
in the order of allocations with no means to change this.

Introduce additional ioctl parameters numValid and num to configure the
line number to allow predictable virtual tty allocation and numbering.
Especially when using multiple n_gsm instances at the same time.

Signed-off-by: Daniel Starke <daniel.starke@...mens.com>
---
 drivers/tty/n_gsm.c         | 85 +++++++++++++++++++++++++++++++++++--
 include/uapi/linux/gsmmux.h |  4 +-
 2 files changed, 85 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 5e516f5cac5a..fafef4edea4f 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -228,7 +228,7 @@ struct gsm_mux {
 	struct tty_struct *tty;		/* The tty our ldisc is bound to */
 	spinlock_t lock;
 	struct mutex mutex;
-	unsigned int num;
+	unsigned short num;
 	struct kref ref;
 
 	/* Events on the GSM channel */
@@ -519,6 +519,64 @@ static void gsm_hex_dump_bytes(const char *fname, const u8 *data,
 	kfree(prefix);
 }
 
+/**
+ * gsm_get_mux	-	find a mux instance by line number
+ *
+ * @num:    the mux line number to search for may differ from the position
+ *          in the mux array.
+ *
+ * @return: Pointer of the mux instance with index <index> or
+ *          NULL if no instance with that index exists
+ */
+static struct gsm_mux *gsm_get_mux(unsigned int num)
+{
+	unsigned int i;
+	struct gsm_mux *gsm;
+
+	/* find the object with that line number */
+	spin_lock(&gsm_mux_lock);
+	for (i = 0, gsm = NULL; i < MAX_MUX; i++) {
+		if (gsm_mux[i]) {
+			if (num == gsm_mux[i]->num) {
+				gsm = gsm_mux[i];
+				break;
+			}
+		}
+	}
+	spin_unlock(&gsm_mux_lock);
+	return gsm;
+}
+
+/**
+ * gsm_assign_num	-	assigns a new mux line number
+ *
+ * @gsm: GSM mux
+ * @num: the desired new mux line number
+ *
+ * The global GSM mux line table is modified to reflect this change.
+ */
+static int gsm_assign_num(struct gsm_mux *gsm, unsigned int num)
+{
+	int ret = 0;
+
+	if (gsm->num >= MAX_MUX || num >= MAX_MUX)
+		return -EINVAL;
+
+	spin_lock(&gsm_mux_lock);
+	if (!gsm_mux[num]) {
+		if (gsm_mux[gsm->num])
+			gsm_mux[gsm->num] = NULL;
+		gsm_mux[num] = gsm;
+		gsm->num = num;
+	} else if (gsm->num != num) {
+		/* Setting an occupied line number is not allowed. */
+		ret = -EBUSY;
+	}
+	spin_unlock(&gsm_mux_lock);
+
+	return ret;
+}
+
 /**
  *	gsm_register_devices	-	register all tty devices for a given mux index
  *
@@ -2700,6 +2758,8 @@ static void gsm_copy_config_values(struct gsm_mux *gsm,
 	c->mru = gsm->mru;
 	c->mtu = gsm->mtu;
 	c->k = 0;
+	c->numValid = 1;
+	c->num = gsm->num;
 }
 
 static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
@@ -2707,6 +2767,7 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 	int ret = 0;
 	int need_close = 0;
 	int need_restart = 0;
+	struct gsm_mux *found;
 
 	/* Stuff we don't support yet - UI or I frame transport, windowing */
 	if ((c->adaption != 1 && c->adaption != 2) || c->k)
@@ -2722,10 +2783,22 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 		return -EINVAL;
 	if (c->i == 0 || c->i > 2)	/* UIH and UI only */
 		return -EINVAL;
+	if (c->numValid) {
+		if (c->num >= MAX_MUX)
+			return -EINVAL;
+		found = gsm_get_mux(c->num);
+		if (found) {
+			/* If there is a mux with that number then it must be the same. */
+			if (found != gsm)
+				return -EBUSY;
+		}
+	}
 	/*
 	 * See what is needed for reconfiguration
 	 */
-
+	/* Line number */
+	if (c->numValid && gsm->num != c->num)
+		need_restart = true;
 	/* Timing fields */
 	if (c->t1 != 0 && c->t1 != gsm->t1)
 		need_restart = 1;
@@ -2748,8 +2821,14 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 	 * configuration. On the first time there is no DLCI[0]
 	 * and closing or cleaning up is not necessary.
 	 */
-	if (need_close || need_restart)
+	if (need_close || need_restart) {
 		gsm_cleanup_mux(gsm, true);
+		if (c->numValid) {
+			ret = gsm_assign_num(gsm, c->num);
+			if (ret)
+				return ret;
+		}
+	}
 
 	gsm->initiator = c->initiator;
 	gsm->mru = c->mru;
diff --git a/include/uapi/linux/gsmmux.h b/include/uapi/linux/gsmmux.h
index cb8693b39cb7..03b28a0076d8 100644
--- a/include/uapi/linux/gsmmux.h
+++ b/include/uapi/linux/gsmmux.h
@@ -19,7 +19,9 @@ struct gsm_config
 	unsigned int mtu;
 	unsigned int k;
 	unsigned int i;
-	unsigned int unused[8];		/* Padding for expansion without
+	unsigned short numValid;
+	unsigned short num;
+	unsigned int unused[7];		/* Padding for expansion without
 					   breaking stuff */
 };
 
-- 
2.34.1

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