Input: add support for Wacom protocol 4 serial tablets

Recent version of xf86-input-wacom no longer support directly accessing
serial tablets. Instead xf86-input-wacom now expects all wacom tablets to
be driven by the kernel and to show up as evdev devices.

This has caused old serial Wacom tablets to stop working for people who still
have such tablets. Julian Squires has written a serio input driver to fix this:
https://github.com/tokenrove/wacom-serial-iv

This is a cleaned up version of this driver with improved Graphire support
(I own an old Graphire myself).
Signed-off-by: Julian Squires <julian@cipht.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

MAINTAINERS

@@ -9708,6 +9708,13 @@ M:	Pierre Ossman <pierre@ossman.eu>
 S:	Maintained
 F:	drivers/mmc/host/wbsd.*
 
+WACOM PROTOCOL 4 SERIAL TABLETS
+M:	Julian Squires <julian@cipht.net>
+M:	Hans de Goede <hdegoede@redhat.com>
+L:	linux-input@vger.kernel.org
+S:	Maintained
+F:	drivers/input/tablet/wacom_serial4.c
+
 WATCHDOG DEVICE DRIVERS
 M:	Wim Van Sebroeck <wim@iguana.be>
 L:	linux-watchdog@vger.kernel.org

drivers/input/tablet/Kconfig

@@ -89,4 +89,14 @@ config TABLET_USB_WACOM
 	  To compile this driver as a module, choose M here: the
 	  module will be called wacom.
 
+config TABLET_SERIAL_WACOM4
+	tristate "Wacom protocol 4 serial tablet support"
+	select SERIO
+	help
+	  Say Y here if you want to use Wacom protocol 4 serial tablets.
+	  E.g. serial versions of the Cintiq, Graphire or Penpartner.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called wacom_serial4.
+
 endif

drivers/input/tablet/Makefile

@@ -11,3 +11,4 @@ obj-$(CONFIG_TABLET_USB_GTCO)	+= gtco.o
 obj-$(CONFIG_TABLET_USB_HANWANG) += hanwang.o
 obj-$(CONFIG_TABLET_USB_KBTAB)	+= kbtab.o
 obj-$(CONFIG_TABLET_USB_WACOM)	+= wacom.o
+obj-$(CONFIG_TABLET_SERIAL_WACOM4) += wacom_serial4.o

include/uapi/linux/serio.h

@@ -76,5 +76,6 @@
 #define SERIO_HAMPSHIRE	0x3b
 #define SERIO_PS2MULT	0x3c
 #define SERIO_TSC40	0x3d
+#define SERIO_WACOM_IV	0x3e
 
 #endif /* _UAPI_SERIO_H */