linux-kernel - Re: [PATCH v9 09/17] arm: tegra20: cpuidle: Handle case where secondary CPU hangs on entering LP2

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Message-ID: <21e3cc35-cc6b-5452-da93-bdaac43716c5@linaro.org>
Date:   Fri, 21 Feb 2020 21:48:56 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Jasper Korten <jja2000@...il.com>,
        David Heidelberg <david@...t.cz>,
        Peter Geis <pgwipeout@...il.com>, linux-pm@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 09/17] arm: tegra20: cpuidle: Handle case where
 secondary CPU hangs on entering LP2

On 21/02/2020 21:21, Dmitry Osipenko wrote:
> 21.02.2020 23:02, Daniel Lezcano пишет:

[ ... ]

>>>>>>> +
>>>>>>> +		/*
>>>>>>> +		 * The primary CPU0 core shall wait for the secondaries
>>>>>>> +		 * shutdown in order to power-off CPU's cluster safely.
>>>>>>> +		 * The timeout value depends on the current CPU frequency,
>>>>>>> +		 * it takes about 40-150us  in average and over 1000us in
>>>>>>> +		 * a worst case scenario.
>>>>>>> +		 */
>>>>>>> +		do {
>>>>>>> +			if (tegra_cpu_rail_off_ready())
>>>>>>> +				return 0;
>>>>>>> +
>>>>>>> +		} while (ktime_before(ktime_get(), timeout));
>>>>>>
>>>>>> So this loop will aggresively call tegra_cpu_rail_off_ready() and retry 3
>>>>>> times. The tegra_cpu_rail_off_ready() function can be called thoushand of times
>>>>>> here but the function will hang 1.5s :/
>>>>>>
>>>>>> I suggest something like:
>>>>>>
>>>>>> 	while (retries--i && !tegra_cpu_rail_off_ready()) 
>>>>>> 		udelay(100);
>>>>>>
>>>>>> So <retries> calls to tegra_cpu_rail_off_ready() and 100us x <retries> maximum
>>>>>> impact.
>>>>> But udelay() also results into CPU spinning in a busy-loop, and thus,
>>>>> what's the difference?
>>>>
>>>> busy looping instead of register reads with all the hardware things involved behind.
>>>
>>> Please notice that this code runs only on an older Cortex-A9/A15, which
>>> doesn't support WFE for the delaying, and thus, CPU always busy-loops
>>> inside udelay().
>>>
>>> What about if I'll add cpu_relax() to the loop? Do you think it it could
>>> have any positive effect?
>>
>> I think udelay() has a call to cpu_relax().
> 
> Yes, my point is that udelay() doesn't bring much benefit for us here
> because:
> 
> 1. we want to enter into power-gated state as quick as possible and
> udelay() just adds an unnecessary delay
> 
> 2. udelay() spins in a busy-loop until delay is expired, just like we're
> doing it in this function already

In this case why not remove ktime_get() and increase the number of retries?

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