linux-kernel - Re: [PATCH 2/2] time-riscv: Stop stimecmp when cpu hotplug

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Message-ID: <CAK9=C2VbF2JKSfJLVEV4SYQFxr+gmpBTjtK6DuVFSbEAjRmFOA@mail.gmail.com>
Date: Thu, 29 Aug 2024 12:19:34 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Nick Hu <nick.hu@...ive.com>
Cc: greentime.hu@...ive.com, zong.li@...ive.com, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Pavel Machek <pavel@....cz>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Andrew Jones <ajones@...tanamicro.com>, 
	Conor Dooley <conor.dooley@...rochip.com>, Samuel Holland <samuel.holland@...ive.com>, 
	Sunil V L <sunilvl@...tanamicro.com>, linux-pm@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] time-riscv: Stop stimecmp when cpu hotplug

On Thu, Aug 29, 2024 at 11:53 AM Nick Hu <nick.hu@...ive.com> wrote:
>
> Hi Anup
>
> On Thu, Aug 29, 2024 at 1:18 PM Anup Patel <apatel@...tanamicro.com> wrote:
> >
> > On Thu, Aug 29, 2024 at 9:10 AM Nick Hu <nick.hu@...ive.com> wrote:
> > >
> > > Stop the stimecmp when the cpu is going to be off otherwise the timer
> > > interrupt may pending while performing power down operation.
> > >
> > > Signed-off-by: Nick Hu <nick.hu@...ive.com>
> > > ---
> > >  drivers/clocksource/timer-riscv.c | 22 +++++++++++++++-------
> > >  1 file changed, 15 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> > > index 48ce50c5f5e6..9a6acaa8dfb0 100644
> > > --- a/drivers/clocksource/timer-riscv.c
> > > +++ b/drivers/clocksource/timer-riscv.c
> > > @@ -32,15 +32,19 @@
> > >  static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
> > >  static bool riscv_timer_cannot_wake_cpu;
> > >
> > > +static void riscv_clock_stop_stimecmp(void)
> > > +{
> > > +       csr_write(CSR_STIMECMP, ULONG_MAX);
> > > +       if (IS_ENABLED(CONFIG_32BIT))
> > > +               csr_write(CSR_STIMECMPH, ULONG_MAX);
> > > +}
> > > +
> > >  static void riscv_clock_event_stop(void)
> > >  {
> > > -       if (static_branch_likely(&riscv_sstc_available)) {
> > > -               csr_write(CSR_STIMECMP, ULONG_MAX);
> > > -               if (IS_ENABLED(CONFIG_32BIT))
> > > -                       csr_write(CSR_STIMECMPH, ULONG_MAX);
> > > -       } else {
> > > +       if (static_branch_likely(&riscv_sstc_available))
> > > +               riscv_clock_stop_stimecmp();
> > > +       else
> > >                 sbi_set_timer(U64_MAX);
> > > -       }
> > >  }
> > >
> > >  static int riscv_clock_next_event(unsigned long delta,
> > > @@ -126,7 +130,11 @@ static int riscv_timer_starting_cpu(unsigned int cpu)
> > >
> > >  static int riscv_timer_dying_cpu(unsigned int cpu)
> > >  {
> > > -       disable_percpu_irq(riscv_clock_event_irq);
> > > +       if (static_branch_likely(&riscv_sstc_available))
> > > +               riscv_clock_stop_stimecmp();
> > > +       else
> > > +               disable_percpu_irq(riscv_clock_event_irq);
> > > +
> >
> > Not disabling riscv_clock_event_irq here for Sstc would now
> > cause riscv_timer_starting_cpu() to unnecessarily enable it
> > when the CPU is powered-up.
> >
> > I think the below change is sufficient for this patch:
> >
> > diff --git a/drivers/clocksource/timer-riscv.c
> > b/drivers/clocksource/timer-riscv.c
> > index 48ce50c5f5e6..546fd248f4ff 100644
> > --- a/drivers/clocksource/timer-riscv.c
> > +++ b/drivers/clocksource/timer-riscv.c
> > @@ -127,6 +127,11 @@ static int riscv_timer_starting_cpu(unsigned int cpu)
> >  static int riscv_timer_dying_cpu(unsigned int cpu)
> >  {
> >         disable_percpu_irq(riscv_clock_event_irq);
> > +       /*
> > +        * Stop the timer when the cpu is going to be offline otherwise
> > +        * the timer interrupt may be pending while performing power-down.
> > +        */
> > +       riscv_clock_event_stop();
> >         return 0;
> >  }
> >
> The sbi_exit() of OpenSBI will disable mtimecmp when
> sbi_hsm_hart_stop() so the mtimecmp will be disabled twice if there is
> no SSTC.
> How about adding a SSTC available check before the riscv_clock_event_stop?

Currently, OpenSBI uses mtimecmp only for implementing
SBI time extension but in the future, OpenSBI might implement
a per-hart timer event list to allow M-mode timer events.

Don't assume in what environment the driver is running.

It is possible that the driver is running under a hypervisor
which only provides SBI time extension and does not virtualize
Sstc extension.

>
>   /*
>    * Stop the timer when the cpu is going to be offline otherwise
>    * the timer interrupt may be pending while performing power-down.
>    */
> if (static_branch_likely(&riscv_sstc_available))
>     riscv_clock_event_stop();
>

Regards,
Anup