linux-kernel - Re: [PATCH v4 2/5] ARC: clocksource: DT based probe

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Message-ID: <570F63A1.3070904@arm.com>
Date:	Thu, 14 Apr 2016 10:32:17 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Noam Camus <noamc@...hip.com>
Subject: Re: [PATCH v4 2/5] ARC: clocksource: DT based probe

On 14/04/16 10:26, Vineet Gupta wrote:
> On Wednesday 13 April 2016 09:52 PM, Marc Zyngier wrote:
>>> -int arc_counter_setup(void)
>>>> +static void __init arc_cs_setup_rtc(struct device_node *node)
>>>>  {
>>>> -	write_aux_reg(ARC_REG_TIMER1_LIMIT, ARC_TIMER_MAX);
>>>> -	write_aux_reg(ARC_REG_TIMER1_CNT, 0);
>>>> -	write_aux_reg(ARC_REG_TIMER1_CTRL, TIMER_CTRL_NH);
>>>> +	int exists = cpuinfo_arc700[smp_processor_id()].extn.rtc;
>>>> +
>>>> +	if (WARN(!exists, "Local-64-bit-Ctr clocksource not detected"))
>>>> +		return;
>>>> +
>>>> +	/* Local to CPU hence not usable in SMP */
>>>> +	if (WARN(IS_ENABLED(CONFIG_SMP), "Local-64-bit-Ctr not usable in SMP"))
>>>> +		return;
>> Sorry if this outlines my lack of understanding of the ARC architecture,
>> but what makes per-cpu timer unsuitable for SMP? I'd have thought that
>> it was actually what you wanted...
> 
> This is clocksource, not clockevent. cs needs to synchronized across all cores so
> that concurrent gtod call from threads on different cores gives you similar
> values. This obviously is not true for the local RTC hardware timer.

Unsynchronized counters on SMP HW, who would have thought! ;-) I guess
each and every architecture has to repeat the same mistakes.

Thanks for shedding some light on it.

	M.
-- 
Jazz is not dead. It just smells funny...