linux-kernel - Re: [PATCH 06/12] time: Add infrastructure to cap clocksource reads to the max

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Message-ID: <20150123142825.GB23123@twins.programming.kicks-ass.net>
Date:	Fri, 23 Jan 2015 15:28:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 06/12] time: Add infrastructure to cap clocksource reads
 to the max_cycles value

On Thu, Jan 22, 2015 at 04:09:21PM -0800, John Stultz wrote:
> +static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr)
> +{
> +	cycle_t cycle_now, delta;
> +
> +	/* read clocksource */
> +	cycle_now = tkr->read(tkr->clock);
> +
> +	/* calculate the delta since the last update_wall_time */
> +	delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask);
> +
> +	/* Cap delta value to the max_cycles values to avoid mult overflows */
> +	if (unlikely(delta > tkr->clock->max_cycles))
> +		delta = tkr->clock->max_cycles;


Should we not raise _something_ here? I know we cannot printk() here,
but bad (TM) things happened if this ever triggers.

> +
> +	return delta;
> +}
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