Re: [PATCH v4 2/3] dt-bindings: timer: Add bindings for the RISC-V timer device

[Anup Patel <anup@brainfault.org>]

On Wed, Nov 30, 2022 at 10:15 AM Samuel Holland <samuel@sholland.org> wrote:
>
> On 11/29/22 08:03, Anup Patel wrote:
> > We add DT bindings for a separate RISC-V timer DT node which can
> > be used to describe implementation specific behaviour (such as
> > timer interrupt not triggered during non-retentive suspend).
> >
> > Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> > Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
> > ---
> >  .../bindings/timer/riscv,timer.yaml           | 52 +++++++++++++++++++
> >  1 file changed, 52 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/timer/riscv,timer.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/timer/riscv,timer.yaml b/Documentation/devicetree/bindings/timer/riscv,timer.yaml
> > new file mode 100644
> > index 000000000000..cf53dfff90bc
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/timer/riscv,timer.yaml
> > @@ -0,0 +1,52 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/timer/riscv,timer.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: RISC-V timer
> > +
> > +maintainers:
> > +  - Anup Patel <anup@brainfault.org>
> > +
> > +description: |+
> > +  RISC-V platforms always have a RISC-V timer device for the supervisor-mode
> > +  based on the time CSR defined by the RISC-V privileged specification. The
> > +  timer interrupts of this device are configured using the RISC-V SBI Time
> > +  extension or the RISC-V Sstc extension.
> > +
> > +  The clock frequency of RISC-V timer device is specified via the
> > +  "timebase-frequency" DT property of "/cpus" DT node which is described
> > +  in Documentation/devicetree/bindings/riscv/cpus.yaml
> > +
> > +properties:
> > +  compatible:
> > +    enum:
> > +      - riscv,timer
> > +
> > +  interrupts-extended:
> > +    minItems: 1
> > +    maxItems: 4096   # Should be enough?
> > +
> > +  riscv,timer-cant-wake-cpu:
>
> I don't want to derail getting this merged, but if you do end up sending
> another version, could you please spell out the word "cannot" here and
> in the code? The missing apostrophe makes this jarring (and an entirely
> different word).

Okay, I will update.

>
> > +    type: boolean
> > +    description:
> > +      If present, the timer interrupt can't wake up the CPU from
> > +      suspend/idle state.
>
> And in that case I would also suggest clarifying this as "one or more
> suspend/idle states", since the limitation does not apply to all idle
> states. At least it should never apply to the architectural WFI state;
> for the SBI idle state binding, it only applies to those with the
> "local-timer-stop" property.

Okay, I will update.

>
> > +
> > +additionalProperties: false
> > +
> > +required:
> > +  - compatible
> > +  - interrupts-extended
> > +
> > +examples:
> > +  - |
> > +    timer {
> > +      compatible = "riscv,timer";
> > +      interrupts-extended = <&cpu1intc 5>,
> > +                            <&cpu2intc 5>,
> > +                            <&cpu3intc 5>,
> > +                            <&cpu4intc 5>;
>
> The CLINT and PLIC bindings also include the M-mode interrupts. Should
> we do the same here?

The RISC-V timer uses SBI time extension or RISC-V Sstc extension hence
it is only for S-mode software. In other words, the RISC-V timer is a S-mode
only timer.

The M-mode software is supposed to have its own platform specific MMIO
based timer.

Regards,
Anup