greybus: uart-gb: improve minor device number error checking

When alloc_minor() finds an available minor device number it
does not constrain the highest number desired.  Instead, it
relies on its caller, tty_gb_probe() to see if the returned
number indicates all minor numbers have been exhausted.

There are a couple problems with this--or rather with this
code.

First, if an allocation is attempted *after* GB_NUM_MINORS
is returned, a new number greater than (but not equal to)
GB_NUM_MINORS will be allocated, and that won't produce
any error condition.

Second, alloc_minor() can return an error code (like -ENOMEM).  And
its caller is only checking for GB_NUM_MINORS.  If an error code
is returned, tty_gb_probe() simply uses it.

Change alloc_minor() so it requests minor device numbers in the
range 0..(GB_NUM_MINORS-1), and use an error return to detect
when the minor device numbers have been exhausted.

If alloc_minor() returns -ENOSPC (from idr_alloc()), translate that
to -ENODEV.  The only other error we might see is -ENOMEM, and if
we get that, return it.

Finally, zero gb_tty->minor when it's released.  (If this is
actually important a reserved value like GB_NUM_MINORS should
be used instead to signify a gb_tty with no minor assigned.)
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/uart-gb.c

@@ -80,19 +80,20 @@ static int alloc_minor(struct gb_tty *gb_tty)
 	int minor;
 
 	mutex_lock(&table_lock);
-	minor = idr_alloc(&tty_minors, gb_tty, 0, 0, GFP_KERNEL);
-	if (minor < 0)
-		goto error;
-	gb_tty->minor = minor;
-error:
+	minor = idr_alloc(&tty_minors, gb_tty, 0, GB_NUM_MINORS, GFP_KERNEL);
 	mutex_unlock(&table_lock);
+	if (minor >= 0)
+		gb_tty->minor = minor;
 	return minor;
 }
 
 static void release_minor(struct gb_tty *gb_tty)
 {
+	int minor = gb_tty->minor;
+
+	gb_tty->minor = 0;	/* Maybe should use an invalid value instead */
 	mutex_lock(&table_lock);
-	idr_remove(&tty_minors, gb_tty->minor);
+	idr_remove(&tty_minors, minor);
 	mutex_unlock(&table_lock);
 }
 
@@ -400,9 +401,12 @@ static int tty_gb_probe(struct greybus_device *gdev, const struct greybus_device
 		return -ENOMEM;
 
 	minor = alloc_minor(gb_tty);
-	if (minor == GB_NUM_MINORS) {
-		dev_err(&gdev->dev, "no more free minor numbers\n");
-		return -ENODEV;
+	if (minor < 0) {
+		if (minor == -ENOSPC) {
+			dev_err(&gdev->dev, "no more free minor numbers\n");
+			return -ENODEV;
+		}
+		return minor;
 	}
 
 	gb_tty->minor = minor;