1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/input/rotary-encoder.txt b/Documentation/input/rotary-encoder.txt
index 943e8f6f2b15..92e68bce13a4 100644
--- a/Documentation/input/rotary-encoder.txt
+++ b/Documentation/input/rotary-encoder.txt
@@ -9,6 +9,9 @@ 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:
 
                   _____       _____       _____
@@ -26,6 +29,8 @@ The phase diagram of these two outputs look like this:
                 |<-------->|
                   one step
 
+                |<-->|
+                  one step (half-period mode)
 
 For more information, please see
         http://en.wikipedia.org/wiki/Rotary_encoder
@@ -34,6 +39,13 @@ For more information, please see
 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
 
@@ -96,6 +108,7 @@ static struct rotary_encoder_platform_data my_rotary_encoder_info = {
         .gpio_b         = GPIO_ROTARY_B,
         .inverted_a     = 0,
         .inverted_b     = 0,
+        .half_period    = false,
 };
 
 static struct platform_device rotary_encoder_device = {