diff --git a/Documentation/input/event-codes.rst b/Documentation/input/event-codes.rst
index b24b5343f5eb..3118fc1c1e26 100644
--- a/Documentation/input/event-codes.rst
+++ b/Documentation/input/event-codes.rst
@@ -236,6 +236,21 @@ A few EV_ABS codes have special meanings:
   - Used to describe multitouch input events. Please see
     multi-touch-protocol.txt for details.
 
+* ABS_PRESSURE/ABS_MT_PRESSURE:
+
+   - For touch devices, many devices converted contact size into pressure.
+     A finger flattens with pressure, causing a larger contact area and thus
+     pressure and contact size are directly related. This is not the case
+     for other devices, for example digitizers and touchpads with a true
+     pressure sensor ("pressure pads").
+
+     A device should set the resolution of the axis to indicate whether the
+     pressure is in measurable units. If the resolution is zero, the
+     pressure data is in arbitrary units. If the resolution is nonzero, the
+     pressure data is in units/gram. For example, a value of 10 with a
+     resolution of 1 represents 10 gram, a value of 10 with a resolution on
+     1000 represents 10 microgram.
+
 EV_SW
 -----
 
diff --git a/Documentation/input/multi-touch-protocol.rst b/Documentation/input/multi-touch-protocol.rst
index 307fe22d9668..21c1e6a22888 100644
--- a/Documentation/input/multi-touch-protocol.rst
+++ b/Documentation/input/multi-touch-protocol.rst
@@ -260,6 +260,10 @@ ABS_MT_PRESSURE
     of TOUCH and WIDTH for pressure-based devices or any device with a spatial
     signal intensity distribution.
 
+    If the resolution is zero, the pressure data is in arbitrary units.
+    If the resolution is nonzero, the pressure data is in units/gram. See
+    :ref:`input-event-codes` for details.
+
 ABS_MT_DISTANCE
     The distance, in surface units, between the contact and the surface. Zero
     distance means the contact is touching the surface. A positive number means