serial: core: add comment about definitely used port types

When port type 18 was removed, it was deduced that the code could go but
its define has to stay because it is used in userspace. Share that
knowledge by adding a comment about it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20230922063642.4120-3-wsa+renesas@sang-engineering.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/uapi/linux/serial_core.h b/include/uapi/linux/serial_core.h
index 92e3dd56f6316295a980ee9e97e50cb30ad42104..46e06c4838999464d17c5901d59bb3e2043a06c6 100644 (file)
--- a/include/uapi/linux/serial_core.h
+++ b/include/uapi/linux/serial_core.h
@@ -11,6 +11,8 @@
  * The type definitions.  These are from Ted Ts'o's serial.h
  * By historical reasons the values from 0 to 13 are defined
  * in the include/uapi/linux/serial.h, do not define them here.
+ * Values 0 to 19 are used by setserial from busybox and must never
+ * be modified.
  */
 #define PORT_NS16550A  14
 #define PORT_XSCALE    15