• Benjamin Tissoires's avatar
    Revert "Input: Add the `REL_WHEEL_HI_RES` event code" · ffe0e7cf
    Benjamin Tissoires authored
    This reverts commit aaf9978c.
    
    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's avatarBenjamin Tissoires <benjamin.tissoires@redhat.com>
    Acked-by: Harry Cutts's avatarHarry Cutts <hcutts@chromium.org>
    Acked-by: default avatarDmitry Torokhov <dmitry.torokhov@gmail.com>
    Acked-by: default avatarJiri Kosina <jkosina@suse.cz>
    ffe0e7cf
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