Add the rt linux 4.1.3-rt3 as base

[kvmfornfv.git] / kernel / Documentation / input / rotary-encoder.txt

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+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.
+
+Some encoders have both outputs low in stable states, whereas others also have
+a stable state with both outputs high (half-period mode).
+
+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
+
+                |<-->|
+                 one step (half-period mode)
+
+For more information, please see
+       http://en.wikipedia.org/wiki/Rotary_encoder
+
+
+1. Events / state machine
+-------------------------
+
+In half-period mode, state a) and c) above are used to determine the
+rotational direction based on the last stable state. Events are reported in
+states b) and d) given that the new stable state is different from the last
+(i.e. the rotation was not reversed half-way).
+
+Otherwise, the following apply:
+
+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 happened, 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 an inverting buffer or other reasons). The encoder
+can be set up to deliver input information as either an absolute or relative
+axes. For relative axes the input event returns +/-1 for each step. For
+absolute axes the position of the encoder can either roll over between zero
+and the number of steps or will clamp at the maximum and zero depending on
+the configuration.
+
+Because GPIO to IRQ mapping is platform specific, this information must
+be given in separately 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,
+       .relative_axis  = false,
+       .rollover       = false,
+       .gpio_a         = GPIO_ROTARY_A,
+       .gpio_b         = GPIO_ROTARY_B,
+       .inverted_a     = 0,
+       .inverted_b     = 0,
+       .half_period    = false,
+};
+
+static struct platform_device rotary_encoder_device = {
+       .name           = "rotary-encoder",
+       .id             = 0,
+       .dev            = {
+               .platform_data = &my_rotary_encoder_info,
+       }
+};
+