Re: [PATCH v2 1/3] dt-bindings: clocks: add binding for voltage-controlled-oscillators

Heiko Stübner
2024-07-18T09:25:41Z

Hi Conor,

Am Dienstag, 16. Juli 2024, 18:15:08 CEST schrieb Conor Dooley:
> On Mon, Jul 15, 2024 at 01:02:49PM +0200, Heiko Stuebner wrote:
> > In contrast to fixed clocks that are described as ungateable, boards
> > sometimes use additional oscillators for things like PCIe reference
> > clocks, that need actual supplies to get enabled and enable-gpios to be
> > toggled for them to work.
> >
> > This adds a binding for such oscillators that are not configurable
> > themself, but need to handle supplies for them to work.
> >
> > In schematics they often can be seen as
> >
> > ----------------
> > Enable - | 100MHz,3.3V, | - VDD
> > | 3225 |
> > GND - | | - OUT
> > ----------------
> >
> > or similar. The enable pin might be separate but can also just be tied
> > to the vdd supply, hence it is optional in the binding.
> >
> > Signed-off-by: Heiko Stuebner <heiko@sntech.de>
> > ---
> > .../bindings/clock/voltage-oscillator.yaml | 49 +++++++++++++++++++
> > 1 file changed, 49 insertions(+)
> > create mode 100644 Documentation/devicetree/bindings/clock/voltage-oscillator.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/clock/voltage-oscillator.yaml b/Documentation/devicetree/bindings/clock/voltage-oscillator.yaml
> > new file mode 100644
> > index 0000000000000..8bff6b0fd582e
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/clock/voltage-oscillator.yaml
> > @@ -0,0 +1,49 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/clock/voltage-oscillator.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Voltage controlled oscillator
>
> Voltage controlled oscillator? Really? That sounds far too similar to a
> VCO to me, and the input voltage here (according to the description at
> least) does not affect the frequency of oscillation.

That naming was suggested by Stephen in v1 [0] .

Of course the schematics for the board I have only describe it as
"100MHz,3.3V,3225" , thumbing through some mouser parts matching that
only mentions "supply voltage" in their datasheets but not a dependency
between rate and voltage.

[0] https://lore.kernel.org/linux-arm-kernel/b3c450a94bcb4ad0bc5b3c7ee8712cb8.sboyd@kernel.org/

> Why the dedicated binding, rather than adding a supply and enable-gpio
> to the existing "fixed-clock" binding? I suspect that a large portion of
> "fixed-clock"s actually require a supply that is (effectively)
> always-on.

I guess there are three aspects:
- I do remember discussions in the past about not extending generic
bindings with device-specific stuff. I think generic power-sequences
were the topic back then, though that might have changed over time?
- There are places that describe "fixed-clock" as
"basic fixed-rate clock that cannot gate" [1]
- Stephen also suggested a separate binding [2]

With the fixed-clock being sort of the root for everything else on most
systems, I opted to leave it alone. I guess if the consenus really is that
this should go there, I can move it, but discussion in v1

Interestingly the fixed clock had a gpios property 10 years ago [3] :-) .

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/clk-fixed-rate.c#n18
[2] https://lore.kernel.org/linux-arm-kernel/68f6dc44a8202fd83792e58aea137632.sboyd@kernel.org/
[3] https://lore.kernel.org/linux-kernel//20140515064420.9521.47383@quantum/T/#t

Heiko

> > +
> > +maintainers:
> > + - Heiko Stuebner <heiko@sntech.de>
> > +
> > +properties:
> > + compatible:
> > + const: voltage-oscillator
> > +
> > + "#clock-cells":
> > + const: 0
> > +
> > + clock-frequency: true
> > +
> > + clock-output-names:
> > + maxItems: 1
> > +
> > + enable-gpios:
> > + description:
> > + Contains a single GPIO specifier for the GPIO that enables and disables
> > + the oscillator.
> > + maxItems: 1
> > +
> > + vdd-supply:
> > + description: handle of the regulator that provides the supply voltage
> > +
> > +required:
> > + - compatible
> > + - "#clock-cells"
> > + - clock-frequency
> > + - vdd-supply
> > +
> > +additionalProperties: false
> > +
> > +examples:
> > + - |
> > + voltage-oscillator {
> > + compatible = "voltage-oscillator";
> > + #clock-cells = <0>;
> > + clock-frequency = <1000000000>;
> > + vdd-supply = <&reg_vdd>;
> > + };
> > +...
>