linux-kernel - Re: [PATCH 2/2] clocksource: timer-riscv: Set CLOCK_EVT_FEAT

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Date:   Thu, 21 Jul 2022 15:15:33 +0530
From:   Anup Patel <apatel@...tanamicro.com>
To:     Samuel Holland <samuel@...lland.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Atish Patra <atishp@...shpatra.org>,
        Anup Patel <anup@...infault.org>, devicetree@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] clocksource: timer-riscv: Set CLOCK_EVT_FEAT_C3STOP
 based on DT property

On Tue, Jul 19, 2022 at 12:12 PM Samuel Holland <samuel@...lland.org> wrote:
>
> Hi Anup,
>
> On 7/19/22 12:47 AM, Anup Patel wrote:
> > We should set CLOCK_EVT_FEAT_C3STOP for a clock_event_device only when
> > riscv,timer-always-on DT property is not present for the corresponding
> > CPU.
>
> The timer maintaining its context (and continuing to count) during non-retentive
> CPU suspend is not sufficient to drop CLOCK_EVT_FEAT_C3STOP.
>
> Another requirement is that the timer interrupt is generated and routed outside
> the CPU's power/reset domain, to whatever hardware is responsible for turning
> the CPU back on. It does not matter if the timer interrupt fires, if that
> interrupt cannot wake up the CPU.
>
> So something closer to "riscv,timer-can-wake-cpu" would be a more accurate
> property name for how you are using it.

I agree with your suggestion. Let's make this DT property name aligned with
what it is doing.

>
> And even then, that ability is a property of the SBI implementation, not just
> the hardware. In the motivating example for the flag (Allwinner D1), the CLINT
> cannot wake the CPU from reset, but the SoC contains other MMIO timers that can.
> So the capability of the SBI timer extension depends on which timer hardware the
> SBI implementation chooses to use. So I am not sure that the property belongs in
> the CPU node.
>
> Maybe it makes sense to report this capability via a function in the SBI timer
> extension?

Well, the timer interrupt not firing in non-retentive suspend is an attribute of
underlying platform and not of the SBI implementation (firmware/hypervisor)
hence should be described in DT or ACPI.

Also, the timer interrupt not firing in non-retentive suspend is also possible
with RISC-V Sstc extension (i.e stimecmp CSRs) so the proposed DT
property will be useful for platforms with Sstc extension as well.

Regards,
Anup

>
> Regards,
> Samuel
>
> > This way CLOCK_EVT_FEAT_C3STOP feature is set for clock_event_device
> > based on RISC-V platform capabilities rather than having it set for
> > all RISC-V platforms.
> >
> > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> > ---
> >  drivers/clocksource/timer-riscv.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> > index 593d5a957b69..3015324f2b59 100644
> > --- a/drivers/clocksource/timer-riscv.c
> > +++ b/drivers/clocksource/timer-riscv.c
> > @@ -34,7 +34,7 @@ static int riscv_clock_next_event(unsigned long delta,
> >  static unsigned int riscv_clock_event_irq;
> >  static DEFINE_PER_CPU(struct clock_event_device, riscv_clock_event) = {
> >       .name                   = "riscv_timer_clockevent",
> > -     .features               = CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_C3STOP,
> > +     .features               = CLOCK_EVT_FEAT_ONESHOT,
> >       .rating                 = 100,
> >       .set_next_event         = riscv_clock_next_event,
> >  };
> > @@ -65,9 +65,13 @@ static struct clocksource riscv_clocksource = {
> >  static int riscv_timer_starting_cpu(unsigned int cpu)
> >  {
> >       struct clock_event_device *ce = per_cpu_ptr(&riscv_clock_event, cpu);
> > +     struct device_node *np = of_get_cpu_node(cpu, NULL);
> >
> >       ce->cpumask = cpumask_of(cpu);
> >       ce->irq = riscv_clock_event_irq;
> > +     if (!of_property_read_bool(np, "riscv,timer-always-on"))
> > +             ce->features |= CLOCK_EVT_FEAT_C3STOP;
> > +     of_node_put(np);
> >       clockevents_config_and_register(ce, riscv_timebase, 100, 0x7fffffff);
> >
> >       enable_percpu_irq(riscv_clock_event_irq,
> >
>