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