[PATCH] spi: ads7846 driver

This is a driver for the ADS7846 touchscreen sensor, derived from
the corgi_ts and omap_ts drivers.  Key differences from those two:

  - Uses the new SPI framework (minimalist version)
  - <linux/spi/ads7846.h> abstracts board-specific touchscreen info
  - Sysfs attributes for the temperature and voltage sensors
  - Uses fewer ARM-specific IRQ primitives

The temperature and voltage sensors show up in sysfs like this:

  $ pwd
  /sys/devices/platform/omap-uwire/spi2.0
  $ ls
  bus@          input:event0@ power/        temp1         vbatt
  driver@       modalias      temp0         vaux
  $ cat modalias
  ads7846
  $ cat temp0
  991
  $ cat temp1
  1177
  $

So far only basic testing has been done.  There's a fair amount of hardware
that uses this sensor, and which also runs Linux, which should eventually
be able to use this driver.

One portability note may be of special interest.  It turns out that not all
SPI controllers are happy issuing requests that do things like "write 8 bit
command, read 12 bit response".  Most of them seem happy to handle various
word sizes, so the issue isn't "12 bit response" but rather "different rx
and tx write sizes", despite that being a common MicroWire convention.  So
this version of the driver no longer reads 12 bit native-endian words; it
reads 16-bit big-endian responses, then byteswaps them and shifts the
results to discard the noise.
Signed-off-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/input/touchscreen/Kconfig

@@ -11,6 +11,19 @@ menuconfig INPUT_TOUCHSCREEN
 
 if INPUT_TOUCHSCREEN
 
+config TOUCHSCREEN_ADS7846
+	tristate "ADS 7846 based touchscreens"
+	depends on SPI_MASTER
+	help
+	  Say Y here if you have a touchscreen interface using the
+	  ADS7846 controller, and your board-specific initialization
+	  code includes that in its table of SPI devices.
+
+	  If unsure, say N (but it's safe to say "Y").
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called ads7846.
+
 config TOUCHSCREEN_BITSY
 	tristate "Compaq iPAQ H3600 (Bitsy) touchscreen"
 	depends on SA1100_BITSY

drivers/input/touchscreen/Makefile

@@ -4,6 +4,7 @@
 
 # Each configuration option enables a list of files.
 
+obj-$(CONFIG_TOUCHSCREEN_ADS7846)	+= ads7846.o
 obj-$(CONFIG_TOUCHSCREEN_BITSY)	+= h3600_ts_input.o
 obj-$(CONFIG_TOUCHSCREEN_CORGI)	+= corgi_ts.o
 obj-$(CONFIG_TOUCHSCREEN_GUNZE)	+= gunze.o

include/linux/spi/ads7846.h

+/* linux/spi/ads7846.h */
+
+/* Touchscreen characteristics vary between boards and models.  The
+ * platform_data for the device's "struct device" holts this information.
+ *
+ * It's OK if the min/max values are zero.
+ */
+struct ads7846_platform_data {
+	u16	model;			/* 7843, 7845, 7846. */
+	u16	vref_delay_usecs;	/* 0 for external vref; etc */
+	u16	x_plate_ohms;
+	u16	y_plate_ohms;
+
+	u16	x_min, x_max;
+	u16	y_min, y_max;
+	u16	pressure_min, pressure_max;
+};
+