linux-kernel - Re: regression in ktime.h circa 3.16.0-rc5+ breaks lirc irsend, bad commit 166afb64511

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Message-ID: <CALAqxLVTUSmw=zvyLde3BRfQMFSmePgXoFq+yD-pVFd1zkua6Q@mail.gmail.com>
Date:	Thu, 30 Apr 2015 09:35:04 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Trevor Cordes <trevor@...nopolis.ca>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Josh Boyer <jwboyer@...hat.com>
Subject: Re: regression in ktime.h circa 3.16.0-rc5+ breaks lirc irsend, bad
 commit 166afb64511

On Wed, Apr 29, 2015 at 9:27 PM, Trevor Cordes <trevor@...nopolis.ca> wrote:
> Sorry for the top-posting; Josh Boyer suggested I re-mail this mail
> from last month which didn't get any replies.  I'm still having this
> weird kernel bug affecting me and I've bisected it down to like 2-4
> lines of code.  (I've thought more about my theory regarding
> unsigned/signed below and it's probably wrong, so ignore my
> prognosticating.)  Please see my rhbz link near the bottom for the full
> details.

Thanks so much for the report and all the effort to chase down this regression!

>From your description it does seem like some sort of edge case problem
w/ the 32bit ktime_divns(), but I don't see it right off, and I agree
with Alan to do both calculations and print out warn when that
happens.

There's also not a ton of users of that function, but ktime_us_delta()
is used in drivers/media/rc/ir-lirc-codec.c, which makes use of it in
ir_lirc_transmit_ir().

We should instrument that to see if its calculating negative deltas.

I'll send you a debug patch to do the above.

thanks
-john
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