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SubjectRe: [PATCH v9 09/17] arm: tegra20: cpuidle: Handle case where secondary CPU hangs on entering LP2
On Fri, Feb 21, 2020 at 07:56:51PM +0300, Dmitry Osipenko wrote:
> Hello Daniel,
>
> 21.02.2020 18:43, Daniel Lezcano пишет:
> > On Thu, Feb 13, 2020 at 02:51:26AM +0300, Dmitry Osipenko wrote:
> >> It is possible that something may go wrong with the secondary CPU, in that
> >> case it is much nicer to get a dump of the flow-controller state before
> >> hanging machine.
> >>
> >> Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
> >> Tested-by: Peter Geis <pgwipeout@gmail.com>
> >> Tested-by: Jasper Korten <jja2000@gmail.com>
> >> Tested-by: David Heidelberg <david@ixit.cz>
> >> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> >> ---

[ ... ]

> >> +static int tegra20_wait_for_secondary_cpu_parking(void)
> >> +{
> >> + unsigned int retries = 3;
> >> +
> >> + while (retries--) {
> >> + ktime_t timeout = ktime_add_ms(ktime_get(), 500);
> >
> > Oops I missed this one. Do not use ktime_get() in this code path, use jiffies.
>
> Could you please explain what benefits jiffies have over the ktime_get()?

ktime_get() is very slow, jiffies is updated every tick.

> >> +
> >> + /*
> >> + * 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.

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