linux-kernel - Re: [PATCH v2 1/2] dt-bindings: riscv: Add optional DT property riscv,timer-can-wake-cpu

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Date:   Wed, 27 Jul 2022 17:51:05 +0530
From:   Anup Patel <apatel@...tanamicro.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Jones <ajones@...tanamicro.com>,
        Atish Patra <atishp@...shpatra.org>,
        Samuel Holland <samuel@...lland.org>,
        Anup Patel <anup@...infault.org>, devicetree@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: riscv: Add optional DT property riscv,timer-can-wake-cpu

On Wed, Jul 27, 2022 at 5:37 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 27/07/2022 13:43, Anup Patel wrote:
> > We add an optional DT property riscv,timer-can-wake-cpu which if present
> > in CPU DT node then CPU timer is always powered-on and never loses context.
> >
> > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> > ---
> >  Documentation/devicetree/bindings/riscv/cpus.yaml | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml b/Documentation/devicetree/bindings/riscv/cpus.yaml
> > index d632ac76532e..b60b64b4113a 100644
> > --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
> > +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
> > @@ -78,6 +78,12 @@ properties:
> >        - rv64imac
> >        - rv64imafdc
> >
> > +  riscv,timer-can-wake-cpu:
> > +    type: boolean
> > +    description:
> > +      If present, the timer interrupt can wake up the CPU from
> > +      suspend/idle state.
>
> Isn't this a property of a timer, not CPU? IOW, your timer node should
> have "wakeup-source" property.

Historically (since the early days), we never had a timer node in the
RISC-V world.

>
> Now that's actual problem: why the RISC-V timer is bound to "riscv"
> compatible, not to dedicated timer node? How is it related to actual CPU
> (not SoC)?

The RISC-V timer is always present on all RISC-V platforms because
the "time" CSR is defined by RISC-V privileged specification. The method
to program per-CPU timer events in either using SBI call or Sstc CSRs.

Since, there is no dedicated timer node, we use CPU compatible string
for probing the per-CPU timer.

Regards,
Anup