linux-kernel - Re: [PATCH] clockevents: Sanitize ticks to nsec conversion

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Date:	Thu, 19 Sep 2013 12:15:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
cc:	Ludovic Desroches <ludovic.desroches@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Marc Kleine-Budde <mkl@...gutronix.de>,
	nicolas.ferre@...el.com, LKML <linux-kernel@...r.kernel.org>,
	Marc Pignat <marc.pignat@...s.ch>, john.stultz@...aro.org,
	kernel@...gutronix.de, Ronald Wahl <ronald.wahl@...itan.com>,
	LAK <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clockevents: Sanitize ticks to nsec conversion

On Thu, 19 Sep 2013, Uwe Kleine-König wrote:
> On Thu, Sep 19, 2013 at 12:01:25AM +0200, Thomas Gleixner wrote:
> > Versus the 64bit overflow check, we need to be even more careful. We
> > need to check for overflowing (1 << 63) - 1 (i.e. the max positive
> > value which fits into a s64). See clockevents_program_event().
> 
> That is because you interpret times < 0 as in the past, right? But note
> that the interim result we're talking about here is still to be divided
> by evt->mult. So assuming mult > 1, that check is too strict unless you
> move it below the do_div in clockevent_delta2ns. For sure it makes sense
> to use the same value for a and b in the handling:

No, it's not too strict.

    nsec = (latch << shift) / mult;

Now the backwards conversion does:

    latch = (nsec * mult) >> shift;
 
So we want nsec * mult to be in the positive range of s64. Which
means, that latch << shift must be in that range as well.

Thanks,

	tglx