linux-kernel - Re: [PATCH v3 1/2] dt-bindings: vm-wdt: Add qemu,vm-watchdog compatible

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Message-ID: <Ymqksfth+sj5JOWo@google.com>
Date:   Thu, 28 Apr 2022 14:29:05 +0000
From:   Sebastian Ene <sebastianene@...gle.com>
To:     Rob Herring <robh@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Derek Kiernan <derek.kiernan@...inx.com>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org,
        qperret@...gle.com, will@...nel.org, maz@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 1/2] dt-bindings: vm-wdt: Add qemu,vm-watchdog
 compatible

On Mon, Apr 25, 2022 at 01:29:51PM -0500, Rob Herring wrote:
> On Mon, Apr 25, 2022 at 01:42:05PM +0000, Sebastian Ene wrote:
> > The stall detection mechanism allows to configure the expiration
> > duration and the internal counter clock frequency measured in Hz.
> > Add these properties in the schema.
> > 
> > Signed-off-by: Sebastian Ene <sebastianene@...gle.com>
> > ---
> >  .../devicetree/bindings/misc/vm-wdt.yaml      | 44 +++++++++++++++++++
> >  1 file changed, 44 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/misc/vm-wdt.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/misc/vm-wdt.yaml b/Documentation/devicetree/bindings/misc/vm-wdt.yaml
> > new file mode 100644
> > index 000000000000..cb7665a0c5af
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/misc/vm-wdt.yaml
> > @@ -0,0 +1,44 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/misc/vm-wdt.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: VM watchdog
> > +
> > +description: |
> > +  This binding describes a CPU stall detector mechanism for virtual cpus.
> > +
> > +maintainers:
> > +  - Sebastian Ene <sebastianene@...gle.com>
> > +
> > +properties:
> > +  compatible:
> > +    enum:
> > +      - qemu,vm-watchdog
> > +  clock:

Hi,

> 
> 'clocks' is already a defined property and 'clock' is too close. It's 
> also ambiguous what it is. 'clock-frequency' instead perhaps.
> 

Yes, I think 'clock-frequency' is a better name. I will update it.

> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description: |
> > +      The watchdog internal clock measure in Hz used to decrement the
> > +      watchdog counter register on each tick.
> > +      Defaults to 10 if unset.
> > +  timeout-sec:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description: |
> > +      The watchdog expiration timeout measured in seconds.
> > +      Defaults to 8 if unset.
> > +
> > +required:
> > +  - compatible
> > +
> > +additionalProperties: false
> > +
> > +examples:
> > +  - |
> > +    watchdog {
> > +      compatible = "qemu,vm-watchdog";
> > +      clock = <10>;
> > +      timeout-sec = <8>;
> 
> How does one access this 'hardware'?
> 

This is a MMIO device.

> Why does this need to be in DT?
> 
> We have DT because h/w designers are incapable of making h/w 
> discoverable. Why repeat that problem with s/w interfaces?
> 

We need to have this one in the DT because in a secure VM we only load
trusted DT components. 

> Rob

Thanks,
Sebastian