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

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Message-ID: <alpine.LFD.2.11.1505011421340.7750@knanqh.ubzr>
Date:	Fri, 1 May 2015 14:29:52 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	John Stultz <john.stultz@...aro.org>
cc:	Trevor Cordes <trevor@...nopolis.ca>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Josh Boyer <jwboyer@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: regression in ktime.h circa 3.16.0-rc5+ breaks lirc irsend, bad
 commit 166afb64511

On Fri, 1 May 2015, John Stultz wrote:

> So yea. I see the issue now. The ktime_divns() function uses do_div
> which is an unsigned divide. So negative numbers look like large
> numbers, and it doesn't do sign-extension, so when it divides we lose
> the sign bit.

That doesn't stop there. On 32-bit systems, if the divisor is not 
constant or too large, then __ktime_divns() is used. If you look at the 
code for __ktime_divns(), you'll notice it would get into an infinite 
loop if you pass it a negative divisor.


Nicolas
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