Input: add `REL_WHEEL_HI_RES` and `REL_HWHEEL_HI_RES`

This event code represents scroll reports from high-resolution wheels and
is modelled after the approach Windows uses. The value 120 is one detent
(wheel click) of movement. Mice with higher-resolution scrolling can send
fractions of 120 which must be accumulated in userspace. Userspace can either
wait for a full 120 to accumulate or scroll by fractions of one logical scroll
movement as the events come in. 120 was picked as magic number because it has
a high number of integer fractions that can be used by high-resolution wheels.

For more information see
https://docs.microsoft.com/en-us/previous-versions/windows/hardware/design/dn613912(v=vs.85)

These new axes obsolete REL_WHEEL and REL_HWHEEL. The legacy axes are emulated
by the kernel but the most accurate (and most granular) data is available
through the new axes.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Verified-by: Harry Cutts <hcutts@chromium.org>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>

2 files changed, 22 insertions, 1 deletions

diff --git a/Documentation/input/event-codes.rst b/Documentation/input/event-codes.rst
index a8c0873beb95..b24b5343f5eb 100644
--- a/Documentation/input/event-codes.rst
+++ b/Documentation/input/event-codes.rst
@@ -190,7 +190,26 @@ A few EV_REL codes have special meanings:
 * REL_WHEEL, REL_HWHEEL:
 
   - These codes are used for vertical and horizontal scroll wheels,
-    respectively.
+    respectively. The value is the number of detents moved on the wheel, the
+    physical size of which varies by device. For high-resolution wheels
+    this may be an approximation based on the high-resolution scroll events,
+    see REL_WHEEL_HI_RES. These event codes are legacy codes and
+    REL_WHEEL_HI_RES and REL_HWHEEL_HI_RES should be preferred where
+    available.
+
+* REL_WHEEL_HI_RES, REL_HWHEEL_HI_RES:
+
+  - High-resolution scroll wheel data. The accumulated value 120 represents
+    movement by one detent. For devices that do not provide high-resolution
+    scrolling, the value is always a multiple of 120. For devices with
+    high-resolution scrolling, the value may be a fraction of 120.
+
+    If a vertical scroll wheel supports high-resolution scrolling, this code
+    will be emitted in addition to REL_WHEEL or REL_HWHEEL. The REL_WHEEL
+    and REL_HWHEEL may be an approximation based on the high-resolution
+    scroll events. There is no guarantee that the high-resolution data
+    is a multiple of 120 at the time of an emulated REL_WHEEL or REL_HWHEEL
+    event.
 
 EV_ABS
 ------
diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
index 3eb5a4c3d60a..265ef2028660 100644
--- a/include/uapi/linux/input-event-codes.h
+++ b/include/uapi/linux/input-event-codes.h
@@ -716,6 +716,8 @@
  * the situation described above.
  */
 #define REL_RESERVED		0x0a
+#define REL_WHEEL_HI_RES	0x0b
+#define REL_HWHEEL_HI_RES	0x0c
 #define REL_MAX			0x0f
 #define REL_CNT			(REL_MAX+1)