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

[Anup Patel <anup@brainfault.org>]

On Wed, Jul 27, 2022 at 6:05 PM Krzysztof Kozlowski
<krzysztof.kozlowski@linaro.org> wrote:
>
> On 27/07/2022 14:21, Anup Patel wrote:
> > On Wed, Jul 27, 2022 at 5:37 PM Krzysztof Kozlowski
> > <krzysztof.kozlowski@linaro.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@ventanamicro.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
>
> Timer is always present also on ARMv8 (and ARMv7) yet it has its node.
>
> > 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.
>
> Timer is still not part of CPU. Otherwise you are claiming here that CPU
> can wakeup CPU...

The clocksource (i.e. "time" register) is part of the CPU but it is an
alias/copy
of a free running system counter whereas clockevent devices (i.e. "mtimecmp"
or "stimecmp" registers) are tightly coupled-with/part-of-the CPU.

Some of the CPU suspend/idle states may or may not preserve state of
timer registers so we might not get a timer interrupt to wake up the cpu
from idle state.

>
> >
> > Since, there is no dedicated timer node, we use CPU compatible string
> > for probing the per-CPU timer.
>
> Next time you add a properties:
> riscv,saata-can-wake-cpu
> riscv,usb-can-wake-cpu
> riscv,interrupt-controller-can-wake-cpu
>
> and so on and keep explaining that "historically" you did not define
> separate nodes, so thus must be in CPU node.

This is a one-of-case with RISC-V DeviceTree where we are living with
the fact that there is no timer DT node. If we add a timer DT node now
then we have to deal with compatibility for existing platforms.

>
> You need to properly reflect hardware in the DTS instead of such hacks.
>
> Best regards,
> Krzysztof

Regards,
Anup

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