linux-kernel - Re: [PATCH] Input: evdev - use monotonic clock for event timestamps

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Message-ID: <4E8C6B1F.4090904@gmail.com>
Date:	Wed, 05 Oct 2011 15:35:11 +0100
From:	Chase Douglas <chasedouglas@...il.com>
To:	Henrik Rydberg <rydberg@...omail.se>
CC:	Daniel Kurtz <djkurtz@...omium.org>, dmitry.torokhov@...il.com,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: evdev - use monotonic clock for event timestamps

On 10/05/2011 10:23 AM, Henrik Rydberg wrote:
>> I understand your concern about breaking random drivers, and am hoping
>> that someon on this list could indicate whether this is a real concern
>> or not.  To get a better feeling for possible regressions, I checked
>> xf86-input-evdev & -synaptics, and neither uses the evdev timestamp in
>> their current incarnations.  Any idea what else might be a good place
>> to check?
> 
> The input system is used for all sorts of events - switches, for
> instance. The point is that it is nearly impossible to know if
> something will break or not, hence the reluctance to modify interfaces.
> 
>> One option is to make the evdev timestamp clock source a per-driver
>> configuration option (controllable from userspace?).  This sounds like
>> it is doable, but would be significantly more complicated.
>>
>> Another option would be to timestamp with monotonicraw + boottime +
>> sleeptime.  This would be approximately wall clock time, but without
>> ntp and slew adjustments.  But, I fear this would just make the rare
>> driver issue less obvious, since it would only become obvious when the
>> two clock sources started drifting apart.
> 
> I agree, the problem is not really solvable. Dmitry?

We could put it into the -next tree early on in the cycle, and then it
will be in -next for a cycle and in Linus' tree for the real dev cycle.
By that time we would hope any issues would have emerged.

I'm not sure if that is a responsible approach. I agree that the change
would be good, but how sure would we be that nothing would break based
only on testing in development trees?

My personal thoughts are that I doubt it would cause issues. Based on
that gut feel, I would say that this approach is reasonable. However,
I'm just one voice in all this :).

-- Chase
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