Message-Id: <20181121152712.6770-8-benjamin.tissoires@redhat.com>
Date:   Wed, 21 Nov 2018 16:27:12 +0100
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jiri Kosina <jikos@...nel.org>,
        Harry Cutts <hcutts@...omium.org>,
        Peter Hutterer <peter.hutterer@...-t.net>,
        torvalds@...ux-foundation.org
Cc:     Nestor Lopez Casado <nlopezcasad@...itech.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Subject: [PATCH 7/7] Revert "Input: Add the `REL_WHEEL_HI_RES` event code"

This reverts commit aaf9978c3c0291ef3beaa97610bc9c3084656a85.

Quoting Peter:

There is a HID feature report called "Resolution Multiplier"
Described in the "Enhanced Wheel Support in Windows" doc and
the "USB HID Usage Tables" page 30.

http://download.microsoft.com/download/b/d/1/bd1f7ef4-7d72-419e-bc5c-9f79ad7bb66e/wheel.docx
https://www.usb.org/sites/default/files/documents/hut1_12v2.pdf

This was new for Windows Vista, so we're only a decade behind here. I only
accidentally found this a few days ago while debugging a stuck button on a
Microsoft mouse.

The docs above describe it like this: a wheel control by default sends
value 1 per notch. If the resolution multiplier is active, the wheel is
expected to send a value of $multiplier per notch (e.g. MS Sculpt mouse) or
just send events more often, i.e. for less physical motion (e.g. MS Comfort
mouse).

For the latter, you need the right HW of course. The Sculpt mouse has
tactile wheel clicks, so nothing really changes. The Comfort mouse has
continuous motion with no tactile clicks. Similar to the free-wheeling
Logitech mice but without any inertia.

Note that the doc also says that Vista and onwards *always* enable this
feature where available.

An example HID definition looks like this:

       Usage Page Generic Desktop (0x01)
       Usage Resolution Multiplier (0x48)
       Logical Minimum 0
       Logical Maximum 1
       Physical Minimum 1
       Physical Maximum 16
       Report Size 2 # in bits
       Report Count 1
       Feature (Data, Var, Abs)

So the actual bits have values 0 or 1 and that reflects real values 1 or 16.
We've only seen single-bits so far, so there's low-res and hi-res, but
nothing in between.

The multiplier is available for HID usages "Wheel" and "AC Pan" (horiz wheel).
Microsoft suggests that

> Vendors should ship their devices with smooth scrolling disabled and allow
> Windows to enable it. This ensures that the device works like a regular HID
> device on legacy operating systems that do not support smooth scrolling.
(see the wheel doc linked above)

The mice that we tested so far do reset on unplug.

Device Support looks to be all (?) Microsoft mice but nothing else

Not supported:
- Logitech G500s, G303
- Roccat Kone XTD
- all the cheap Lenovo, HP, Dell, Logitech USB mice that come with a
  workstation that I could find don't have it.
- Etekcity something something
- Razer Imperator

Supported:
- Microsoft Comfort Optical Mouse 3000 - yes, physical: 1:4
- Microsoft Sculpt Ergonomic Mouse - yes, physical: 1:12
- Microsoft Surface mouse - yes, physical: 1:4

So again, I think this is really just available on Microsoft mice, but
probably all decent MS mice released over the last decade.

Looking at the hardware itself:

- no noticeable notches in the weel
- low-res: 18 events per 360deg rotation (click angle 20 deg)
- high-res: 72 events per 360deg → matches multiplier of 4

- I can feel the notches during wheel turns
- low-res: 24 events per 360 deg rotation (click angle 15 deg)
  - horiz wheel is tilt-based, continuous output value 1
- high-res: 24 events per 360deg with value 12 → matches multiplier of 12
  - horiz wheel output rate doubles/triples?, values is 3

- It's a touch strip, not a wheel so no notches
- high-res: events have value 4 instead of 1
  a bit strange given that it doesn't actually have notches.

Ok, why is this an issue for the current API? First, because the logitech
multiplier used in Harry's patches looks suspiciously like the Resolution
Multiplier so I think we should assume it's the same thing. Nestor, can you
shed some light on that?

- `REL_WHEEL` is defined as the number of notches, emulated where needed.
- `REL_WHEEL_HI_RES` is the movement of the user's finger in microns.
- `WM_MOUSEWHEEL` (Windows) is is a multiple of 120, defined as "the threshold
  for action to be taken and one such action"
  https://docs.microsoft.com/en-us/windows/desktop/inputdev/wm-mousewheel

If the multiplier is set to M, this means we need an accumulated value of M
until we can claim there was a wheel click. So after enabling the multiplier
and setting it to the maximum (like Windows):
- M units are 15deg rotation → 1 unit is 2620/M micron (see below). This is
  the `REL_WHEEL_HI_RES` value.
  - wheel diameter 20mm: 15 deg rotation is 2.62mm, 2620 micron (pi * 20mm /
    (360deg/15deg))
- For every M units accumulated, send one `REL_WHEEL` event

The problem here is that we've now hardcoded 20mm/15 deg into the kernel and
we have no way of getting the size of the wheel or the click angle into the
kernel.

In userspace we now have to undo the kernel's calculation. If our click angle
is e.g. 20 degree we have to undo the (lossy) calculation from the kernel and
calculate the correct angle instead. This also means the 15 is a hardcoded
option forever and cannot be changed.

In hid-logitech-hidpp.c, the microns per unit is hardcoded per device.
Harry, did you measure those by hand? We'd need to update the kernel for
every device and there are 10 years worth of devices from MS alone.

The multiplier default is 8 which is in the right ballpark, so I'm pretty
sure this is the same as the Resolution Multiplier, just in HID++ lingo. And
given that the 120 magic factor is what Windows uses in the end, I can't
imagine Logitech rolling their own thing here. Nestor?

And we're already fairly inaccurate with the microns anyway. The MX Anywhere
2S has a click angle of 20 degrees (18 stops) and a 17mm wheel, so a wheel
notch is approximately 2.67mm, one event at multiplier 8 (1/8 of a notch)
would be 334 micron. That's only 80% of the fallback value of 406 in the
kernel. Multiplier 6 gives us 445micron (10% off). I'm assuming multiplier 7
doesn't exist because it's not a factor of 120.

Summary:

Best option may be to simply do what Windows is doing, all the HW manufacturers
have to use that approach after all. Switch `REL_WHEEL_HI_RES` to report in
fractions of 120, with 120 being one notch and divide that by the multiplier
for the actual events. So e.g. the Logitech multiplier 8 would send value 15
for each event in hi-res mode. This can be converted in userspace to
whatever userspace needs (combined with a hwdb there that tells you wheel
size/click angle/...).

Conflicts:
	include/uapi/linux/input-event-codes.h -> I kept the new
         reserved event in the code, so I had to adapt the revert
         slightly

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
---
 Documentation/input/event-codes.rst    | 11 +----------
 include/uapi/linux/input-event-codes.h | 10 ----------
 2 files changed, 1 insertion(+), 20 deletions(-)

diff --git a/Documentation/input/event-codes.rst b/Documentation/input/event-codes.rst
index cef220c176a4..a8c0873beb95 100644
--- a/Documentation/input/event-codes.rst
+++ b/Documentation/input/event-codes.rst
@@ -190,16 +190,7 @@ A few EV_REL codes have special meanings:
 * REL_WHEEL, REL_HWHEEL:
 
   - These codes are used for vertical and horizontal scroll wheels,
-    respectively. The value is the number of "notches" moved on the wheel, the
-    physical size of which varies by device. For high-resolution wheels (which
-    report multiple events for each notch of movement, or do not have notches)
-    this may be an approximation based on the high-resolution scroll events.
-
-* REL_WHEEL_HI_RES:
-
-  - If a vertical scroll wheel supports high-resolution scrolling, this code
-    will be emitted in addition to REL_WHEEL. The value is the (approximate)
-    distance travelled by the user's finger, in microns.
+    respectively.
 
 EV_ABS
 ------
diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
index 6d180cc60a5d..3eb5a4c3d60a 100644
--- a/include/uapi/linux/input-event-codes.h
+++ b/include/uapi/linux/input-event-codes.h
@@ -716,7 +716,6 @@
  * the situation described above.
  */
 #define REL_RESERVED		0x0a
-#define REL_WHEEL_HI_RES	0x0b
 #define REL_MAX			0x0f
 #define REL_CNT			(REL_MAX+1)
 
@@ -753,15 +752,6 @@
 
 #define ABS_MISC		0x28
 
-/*
- * 0x2e is reserved and should not be used in input drivers.
- * It was used by HID as ABS_MISC+6 and userspace needs to detect if
- * the next ABS_* event is correct or is just ABS_MISC + n.
- * We define here ABS_RESERVED so userspace can rely on it and detect
- * the situation described above.
- */
-#define ABS_RESERVED		0x2e
-
 #define ABS_MT_SLOT		0x2f	/* MT slot being modified */
 #define ABS_MT_TOUCH_MAJOR	0x30	/* Major axis of touching ellipse */
 #define ABS_MT_TOUCH_MINOR	0x31	/* Minor axis (omit if circular) */
-- 
2.19.1

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