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

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Message-ID: <CAHk-=widz0eGFuV=bPdXenOEipZg-OTjhfownHUM7gwiZ=4tHw@mail.gmail.com>
Date:   Mon, 26 Nov 2018 18:30:04 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Hutterer <peter.hutterer@...-t.net>
Cc:     linux-input@...r.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jiri Kosina <jikos@...nel.org>,
        Harry Cutts <hcutts@...omium.org>,
        Nestor Lopez Casado <nlopezcasad@...itech.com>,
        Linux List Kernel Mailing <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 3:28 PM Peter Hutterer <peter.hutterer@...-t.net> wrote:
>
> 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.

I think you're just doing the math in the wrong order.

Why don't you just do

    update = val * 120 / multiplier

which gives you the expected "30".

It seems you have done the "120 / multiplier" too early, and you force
that value into "wheel_factor". Don't. Do all the calculations
(including all the accumulated ones) in the original values, and only
do the "multiply by 120 and divide by multiplier" at the very end.

Hmm?

              Linus