linux-kernel - Re: How to handle Elantech touchpad that occasionally sends bogus coordinates?

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Message-ID: <20090305063201.GB14884@dtor-d630.eng.vmware.com>
Date:	Wed, 4 Mar 2009 22:32:01 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Arjan Opmeer <arjan@...eer.net>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: How to handle Elantech touchpad that occasionally sends bogus
	coordinates?

Hi Arjan,

On Wed, Mar 04, 2009 at 10:55:56PM +0100, Arjan Opmeer wrote:
> 
> Recently a user complained that when using the Elantech driver the mouse
> pointer would occasionally jump some distance away, or that when scrolling
> the scrolled page would suddenly jump back a bit.
> 
> It turns out that when starting a new touch action on this particular model
> the touchpad does not reliably emit the new coordinates but can repeat one
> or two of the coordinates of the last release point. This misreporting can
> last several motion packets depending on the number of fingers that are
> touching.
> 
> We came up with a workaround that basically discards mouse packets as long
> as the number of fingers is increasing. Like so:
> 
> 
>    if (fingers > old_fingers) {
>        discard packet
>    }
> 
> With this fix in place the user no longer suffers from the jumpy mouse
> cursor or the jumpy scrolling. He reports no impact on the responsiveness of
> the touchpad.
> 
> My question now is whether this is the right approach to fix this issue? Is
> such a workaround acceptable and should it live in the kernel? Can or should
> this problem be better solved in the Xorg Synaptics driver?
> 

This is purely hardware problem and I believe it should be fixed in the
kernel; userspace should not be aware that device may emit wrong
coordinates.

Thanks!

-- 
Dmitry
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