linux-kernel - Re: [PATCH] usb: generic calibration support

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Date:	Fri, 17 Nov 2006 09:16:05 -0500
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	"Holger Schurig" <hs4233@...l.mn-solutions.de>
Cc:	"Daniel Ritz" <daniel.ritz-ml@...ssonline.ch>, daniel.ritz@....ch,
	linux-kernel@...r.kernel.org, linux-usb-devel@...ts.sourceforge.net
Subject: Re: [PATCH] usb: generic calibration support

Hi,

On 11/17/06, Holger Schurig <hs4233@...l.mn-solutions.de> wrote:
> > sorry, but i have to give you a big NACK on that one:
>
> Hehe, I actually anticipated this NACK :-)
>
> > - no more modparam: it should be per-device sysfs attributes
> > (swap_xy is basically only for touchkitusb compatibility and
> >    shoud be converted to per-device sysfs attribute as well. i
> > just never got to do it)
>
> That would be okay for me.
>

Actually I think having module parameter is OK as long as we have
comlementing device attribute. Then user has an option to either
specify default value for the parameter at load/boot time or fiddle
with an individual device.

> > - calibration can be handled in userspace just fine
>
> Yes it can, and that is the most convinging argument against my
> patch.
>
> However, user-space calibration often sucks. For example, in X11
> the calibration values are stored in /etc/X11/xorg.conf, so a
> calibration program has to parse & write that file. And I've
> seen binary-only-calibration programs from vendors
> use /etc/X11/XF86Config, which doesn't exist here. Anyway, once
> you've calibrated you have to restart X-Windows, so you need to
> terminate all running X applications just because of a silly
> re-calibration. Not nice.
>

I believe tslib handles this.

> A calibration at kernel level (may it be input level or driver
> level) doesn't have this problem. I can make the level
> persistent via udev, modules.conf or other means. Even when I
> have a frame-buffer based calibration utility, the calibration
> would be OK for X11 or GTK/DirectFB as well.
>
> For me, this speaks against "Calibration is purely a user-space
> problem".
>
> > - even for in-kernel it's in the wrong place. there are other
> >   devices that report raw absolute data...so it would belong to
> >   the input layer
>
> Suppose I have a graphics tabled and a touchscreen connected at
> the same time. Can I distinguish this easily at the input layer
> level?
>
> I can very easily distinguish this at the device driver level.
> After all, the individual calibration parameters are a property
> of this individual device.
>

Yes, but evdev is the interface between input layer and userspace. If
we ever implement in-kernel calibration I think we'll add
recalibrate() method to input device structure and have EVIOCSABS
ioctl handler call it. But I'd look at tslib first. I believe Qt,
Xserver, etc support it.

-- 
Dmitry
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