linux-kernel - Re: [PATCH 4/8] HID: input: use the Resolution Multiplier for high-resolution scrolling

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Date:   Fri, 23 Nov 2018 09:28:39 +1000
From:   Peter Hutterer <peter.hutterer@...-t.net>
To:     linux-input@...r.kernel.org
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jiri Kosina <jikos@...nel.org>,
        Harry Cutts <hcutts@...omium.org>,
        torvalds@...ux-foundation.org,
        Nestor Lopez Casado <nlopezcasad@...itech.com>,
        linux-kernel@...r.kernel.org,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Subject: Re: [PATCH 4/8] HID: input: use the Resolution Multiplier for
 high-resolution scrolling

On Thu, Nov 22, 2018 at 04:34:05PM +1000, Peter Hutterer wrote:
> Windows uses a magic number of 120 for a wheel click. High-resolution
> scroll wheels are supposed to use a fraction of 120 to signal smaller
> scroll steps. This is implemented by the Resolution Multiplier in the
> device itself.

I scooped a dirty old MS Wireless Mobile Mouse 4000 out of a mate's bin and
it breaks this assumption. The resolution multiplier is 16 which isn't an
integer fraction of 120. Real multiplier is 7.5.

The device sends hi-res values of 4, so it should end up as REL_WHEEL_HI_RES
30. We are getting 28 instead which doesn't add up to a nice 120.
Basic assumption: MS uses something other than plain 120 internally.

The choices we have now are:
- use 1200 or 12000 internally and divide by 10 before sending the
  final value
- just make the evdev API use 1200 or 12000 and let userspace deal with it.
  much simpler.

Any suggestions/comments?

Cheers,
   Peter