linux-kernel - Re: [PATCH] Input: synaptics - disable fuzz when using mt slots

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Date:	Wed, 13 Feb 2013 13:52:50 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Daniel Kurtz <djkurtz@...omium.org>
Cc:	Henrik Rydberg <rydberg@...omail.se>,
	Seth Forshee <seth.forshee@...onical.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Chung-Yih Wang <cywang@...omium.org>,
	Chase Douglas <chase.douglas@...onical.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: synaptics - disable fuzz when using mt slots

On Fri, Feb 01, 2013 at 04:28:32PM +0800, Daniel Kurtz wrote:
> The input fuzz algorithm does a pretty good job filtering out small
> jitter on input samples.
> 
> However, there is a subtle problem when fuzz is used with mt drivers that
> also use input_mt_report_pointer_emulation() to report legacy single
> touch coordinates.  The reported single touch coordinates are taken from
> the active MT slot with smallest tracking id.  However, since the MT data
> is sent first, it has already had FUZZ applied.  Thus, the ST coordinates
> actually get FUZZ applied a second time, with the just-fuzzed MT value as
> the 'old value'.
> 
> It appears that it is non-trivial to fix this in the mt layer/input core,
> so, for now, just disable fuzz for the synaptics IMAGE_SENSOR path that
> uses input_mt_report_pointer_emulation().
> 
> Signed-off-by: Daniel Kurtz <djkurtz@...omium.org>
> ---
>  drivers/input/mouse/synaptics.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/input/mouse/synaptics.c b/drivers/input/mouse/synaptics.c
> index 12d12ca..b6c6838 100644
> --- a/drivers/input/mouse/synaptics.c
> +++ b/drivers/input/mouse/synaptics.c
> @@ -1211,7 +1211,8 @@ static void set_abs_position_params(struct input_dev *dev,
>  	int x_max = priv->x_max ?: XMAX_NOMINAL;
>  	int y_min = priv->y_min ?: YMIN_NOMINAL;
>  	int y_max = priv->y_max ?: YMAX_NOMINAL;
> -	int fuzz = SYN_CAP_REDUCED_FILTERING(priv->ext_cap_0c) ?
> +	int fuzz = (SYN_CAP_REDUCED_FILTERING(priv->ext_cap_0c) &&
> +			!SYN_CAP_IMAGE_SENSOR(priv->ext_cap_0c)) ?
>  			SYN_REDUCED_FILTER_FUZZ : 0;

Hmm, instead of disabling fuzz completely maybe we should disable it
only on the derived axes (ABS_X/ABS_Y) for multitouch devices?

Thanks.

-- 
Dmitry
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