Input: generic driver for rotary encoders on GPIOs

This patch adds a generic driver for rotary encoders connected to GPIO
pins of a system. It relies on gpiolib and generic hardware irqs. The
documentation that also comes with this patch explains the concept and
how to use the driver.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

1 files changed, 101 insertions, 0 deletions

diff --git a/Documentation/input/rotary-encoder.txt b/Documentation/input/rotary-encoder.txt
new file mode 100644
index 000000000000..435102a26d96
--- /dev/null
+++ b/Documentation/input/rotary-encoder.txt
@@ -0,0 +1,101 @@
+rotary-encoder - a generic driver for GPIO connected devices
+Daniel Mack <daniel@caiaq.de>, Feb 2009
+
+0. Function
+-----------
+
+Rotary encoders are devices which are connected to the CPU or other
+peripherals with two wires. The outputs are phase-shifted by 90 degrees
+and by triggering on falling and rising edges, the turn direction can
+be determined.
+
+The phase diagram of these two outputs look like this:
+
+                  _____       _____       _____
+                 |     |     |     |     |     |
+  Channel A  ____|     |_____|     |_____|     |____
+
+                 :  :  :  :  :  :  :  :  :  :  :  :
+            __       _____       _____       _____
+              |     |     |     |     |     |     |
+  Channel B   |_____|     |_____|     |_____|     |__
+
+                 :  :  :  :  :  :  :  :  :  :  :  :
+  Event          a  b  c  d  a  b  c  d  a  b  c  d
+
+                |<-------->|
+                  one step
+
+
+For more information, please see
+        http://en.wikipedia.org/wiki/Rotary_encoder
+
+
+1. Events / state machine
+-------------------------
+
+a) Rising edge on channel A, channel B in low state
+        This state is used to recognize a clockwise turn
+
+b) Rising edge on channel B, channel A in high state
+        When entering this state, the encoder is put into 'armed' state,
+        meaning that there it has seen half the way of a one-step transition.
+
+c) Falling edge on channel A, channel B in high state
+        This state is used to recognize a counter-clockwise turn
+
+d) Falling edge on channel B, channel A in low state
+        Parking position. If the encoder enters this state, a full transition
+        should have happend, unless it flipped back on half the way. The
+        'armed' state tells us about that.
+
+2. Platform requirements
+------------------------
+
+As there is no hardware dependent call in this driver, the platform it is
+used with must support gpiolib. Another requirement is that IRQs must be
+able to fire on both edges.
+
+
+3. Board integration
+--------------------
+
+To use this driver in your system, register a platform_device with the
+name 'rotary-encoder' and associate the IRQs and some specific platform
+data with it.
+
+struct rotary_encoder_platform_data is declared in
+include/linux/rotary-encoder.h and needs to be filled with the number of
+steps the encoder has and can carry information about externally inverted
+signals (because of used invertig buffer or other reasons).
+
+Because GPIO to IRQ mapping is platform specific, this information must
+be given in seperately to the driver. See the example below.
+
+---------<snip>---------
+
+/* board support file example */
+
+#include <linux/input.h>
+#include <linux/rotary_encoder.h>
+
+#define GPIO_ROTARY_A 1
+#define GPIO_ROTARY_B 2
+
+static struct rotary_encoder_platform_data my_rotary_encoder_info = {
+        .steps          = 24,
+        .axis           = ABS_X,
+        .gpio_a         = GPIO_ROTARY_A,
+        .gpio_b         = GPIO_ROTARY_B,
+        .inverted_a     = 0,
+        .inverted_b     = 0,
+};
+
+static struct platform_device rotary_encoder_device = {
+        .name           = "rotary-encoder",
+        .id             = 0,
+        .dev            = {
+                .platform_data = &my_rotary_encoder_info,
+        }
+};
+