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

[Dragan Simic <dsimic@manjaro.org>]

Hello all,

On 2024-07-18 11:25, Heiko Stübner wrote:
> 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

After finally going through the v1 discussion [4] in detail,
here are my further thoughts:

- I agree with dropping the Diodes stuff, [5] because I see
   no need for that at this point;  though, am I missing
   something, where are they actually used?

- I agree that "enable-gpios" and "vdd-supply" should be
   required in the binding, [5] because that's the basis for
   something to be actually represented this way

- I agree that it should be better not to touch fixed-clock
   at this point, simply because it's used in very many places,
   and because in this case we need more than it requires (see
   the bullet point above)

- As I wrote already, [6] I highly disagree with this being
   called voltage-controlled oscillator (VCO), [7] simply
   because it isn't a VCO, but a clock that can be gated;
   though, looking forward to what the last bullet point
   asks to be clarified further

- I still think that gated-clock is the best choice for the
   name, because it uses "clock" that's used throughout the
   entire codebase, and uses "gated" to reflect the nature
   of the clock generator

- Maybe we could actually use fixed-gated-clock as the name,
   which would make more sense from the stanpoint of possibly
   merging it into fixed-clock at some point, but I'd like
   to hear first what's actually going on with the Diodes
   stuff that was deleted in v2, which I already asked about
   in the first bullet point

[4] 
https://lore.kernel.org/linux-rockchip/20240709123121.1452394-1-heiko@sntech.de/T/#u
[5] 
https://lore.kernel.org/linux-rockchip/2e5852b9e94b9a8d0261ce7ad79f4329.sboyd@kernel.org/
[6] 
https://lore.kernel.org/linux-rockchip/ec84dc37e2c421ee6d31294e08392d57@manjaro.org/
[7] 
https://lore.kernel.org/linux-rockchip/b3c450a94bcb4ad0bc5b3c7ee8712cb8.sboyd@kernel.org/

>> > +
>> > +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>;
>> > +    };
>> > +...