auxdisplay: img-ascii-lcd: driver for simple ASCII LCD displays

Add a driver for simple ASCII LCD displays found on the MIPS Boston,
Malta & SEAD3 development boards. The Boston display is an independent
memory mapped device with a simple memory mapped 8 byte register space
containing the 8 ASCII characters to display. The Malta display is
exposed as part of the Malta board registers, and provides 8 registers
each of which corresponds to one of the ASCII characters to display. The
SEAD3 display is slightly more complex, exposing an interface to an
S6A0069 LCD controller via registers provided by the boards CPLD.
However although the displays differ in their register interface, we
require similar functionality on each board so abstracting away the
differences within a single driver allows us to share a significant
amount of code & ensure consistent behaviour.

The driver displays the Linux kernel version as the default message, but
allows the message to be changed via a character device. Messages longer
then the number of characters that the display can show will scroll.

This provides different behaviour to the existing LCD display code for
the MIPS Malta or MIPS SEAD3 platforms in the following ways:

  - The default string to display is not "LINUX ON MALTA" or "LINUX ON
    SEAD3" but "Linux" followed by the version number of the kernel
    (UTS_RELEASE).

  - Since that string tends to be significantly longer it scrolls twice
    as fast, moving every 500ms rather than every 1s.

  - The LCD won't be updated until the driver is probed, so it doesn't
    provide the early "LINUX" string.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Miguel Ojeda Sandonis <miguel.ojeda.sandonis@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14062/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

MAINTAINERS

@@ -5968,6 +5968,7 @@ IMGTEC ASCII LCD DRIVER
 M:	Paul Burton <paul.burton@imgtec.com>
 S:	Maintained
 F:	Documentation/devicetree/bindings/auxdisplay/img-ascii-lcd.txt
+F:	drivers/auxdisplay/img-ascii-lcd.c
 
 INA209 HARDWARE MONITOR DRIVER
 M:	Guenter Roeck <linux@roeck-us.net>

drivers/auxdisplay/Kconfig

@@ -119,4 +119,13 @@ config CFAG12864B_RATE
 	  If you compile this as a module, you can still override this
 	  value using the module parameters.
 
+config IMG_ASCII_LCD
+	tristate "Imagination Technologies ASCII LCD Display"
+	default y if MIPS_MALTA || MIPS_SEAD3
+	select SYSCON
+	help
+	  Enable this to support the simple ASCII LCD displays found on
+	  development boards such as the MIPS Boston, MIPS Malta & MIPS SEAD3
+	  from Imagination Technologies.
+
 endif # AUXDISPLAY

drivers/auxdisplay/Makefile

@@ -4,3 +4,4 @@
 
 obj-$(CONFIG_KS0108)		+= ks0108.o
 obj-$(CONFIG_CFAG12864B)	+= cfag12864b.o cfag12864bfb.o
+obj-$(CONFIG_IMG_ASCII_LCD)	+= img-ascii-lcd.o