Message-ID: <20120730061114.GE5830@core.coreip.homeip.net>
Date:	Sun, 29 Jul 2012 23:11:14 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	George Spelvin <linux@...izon.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [REPOST #3 PATCH v2] Input: atkbd - make repeat period more
 accurate.

On Thu, Jul 26, 2012 at 11:15:24AM -0400, George Spelvin wrote:
> Thanks for the response!  I'd been checking mailing list archive sites
> to see if my submissions were making it out to the list.
> 
> >> In addition to a slightly inaccurate lookup table, the
> >> old code would round up to the next repeat period.
> >> E.g. to get a period of 9/60 = 0.15 seconds, you had to
> >> ask for no more than 149 ms; if you asked for 150, it
> >> would round up to 167.
> 
> > This works as intended - it was designed to never have faster than
> > requested.
> 
> The old code didn't do *that* correctly, either.  If you asked for 370
> or 470 ms, it would give you 366.66 or 466.66 ms, respectively.
> 
> I can amend the code to do the same, but this leads to some questions:
> 
> Should the reported values be rounded down so that setting the repeat
> period to the reported value is always safe?  Or should it allow rounding
> down by <= 0.5 ms, so that asking for 117 ms will give you 166.66
> without complaint?
> 
> And what do you do if the requested delay or period is longer than can
> be supported (1000 and 500 ms, respectively)?
> 
> Finally, is this rounding documented clearly anywhere?  I thought rounding
> to nearest gave the least surprising results for someone using various
> slightly-inaccurate lists of the repeatable keyboard rates.  Including,
> particularly, the previous code's values.

I do not believe rounding is documented anywhere; the rates are
mentioned in kbdrate manpage.

Frankly at this time I'd just leave this all as is since there were no
complaints from users about repeat rates on keyboards and most clients
(X) implement their own, software-based, rate control.

> 
> >> Signed-off-by: George Spelvin <linux@...izon.com>
> 
> > I am sorry but I have to ask - is this your real name?
> 
> Well, it's meant to be an obvious pen name,

I do not think it is that obvious to anyone who is not living in the US.
The only reason I paid attention to the name is I recognized your e-mail
from LKML as one never signing your name.

> but since I use it
> consistently and live in a common-law country, it's a "real"
> as any other.

I am not sure whether common law countries allow anyone pick any name of
their choosing and use it as their legal name; I know that we had to
refuse submission from another developer wishing to use a pen name
(HDAPS driver).

> 
> >> One possible bug I observed in the code that calls this:
> >> 
> >> Users of the KDKBDREP ioctl seem to assume that it returns the actual
> >> values set, but I'm not sure it really works that way; I don't think
> >> the command to change the parameters makes its way through the event
> >> queue and atkbd's schedule_delayed_work() to actually set dev->rep[]
> >> to the rounded values before kbd_rate_helper returns them to userspace.
> >> 
> >> If desired, the fix that's most obvious to me would be to split this
> >> function in two: perform the conversion to a command byte synchronously,
> >> and only defer the actual ps2_command().
> 
> > Yes, I agree, this is a problem.
> 
> I was trying to start simple, since this is a separate issue,
> but I could make an attempt at this fix, too.
> 
> The *big* problsm is what if there's a peripheral that actually requires
> a USB transaction to set the rate before being able to determine what
> the rounded rate to report is, it'll require a major overhaul of the
> in-kernel interfaces.

Luckily we are using software based repeat on USB (and most other input
devices).

> 
> 
> Another thing that would be possible is supporting an arbitrary repeat
> rate, as long as it's slower than the maximum autorepeat rate.  Just set
> the hardware repeat slightly faster than the software repeat rate and
> buffer the autorepeat reports until the correct software-repeat time.
> 
> I don't know if it's worth bothering, though.

No, I do not think it's worth it as we have software-based repeat
implementation that allows arbitrary rates.

Thanks.

-- 
Dmitry
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