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

Heiko Stübner
2024-07-18T13:50:36Z

Hi Dragan,

Am Donnerstag, 18. Juli 2024, 15:00:57 CEST schrieb Dragan Simic:
> On 2024-07-18 13:30, Heiko Stübner wrote:
> > Am Donnerstag, 18. Juli 2024, 12:53:07 CEST schrieb Dragan Simic:
> >> 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?
> >
> > On the Rock5 ITX that 100MHz clock comes the one single oscillator
> > thing.
> >
> > The Diodes parts are not root sources for their clocks but instead sort
> > PLLs or something, though their manual describes them as
> > "clock generator supporting PCI Express and Ethernet requirements" [8]
> >
> > So they generate the 100MHz (frequency actually is
> > selected by input pins of the chip) from a separate 25MHz source clock.
> >
> > One example are the Theobroma/Cherry embedded boards changed in v1
> > where
> > they currently are described via existing generic things (no
> > schematics).
> >
> > Another user is the rk3568-rock3b for example, where the diodes part
> > is enabled by the same rail as the port itself, so in contrast to the
> > Rock 5 ITX, it works "by accident" on the rock 3b [9]
>
> Ah, I see now, thanks for the clarification. The way Diodes PI6C557
> is used on the ROCK 3B, together with its 25 MHz "passive" crystal,
> is pretty much the same as the way Aurasemi AU5426 is used on the
> ROCK 5 ITX, together with its 100 MHz "active" clock generator. All
> that from the software standpoint, of course.
>
> To explain it further, the PI6C577 and the AU5426 are the components
> that actually generate the clocks for the PCIe interfaces. Thus,
> technically we should describe two components per board in their DTs:
>
> - ROCK 5 ITX:
> - 100 MHz clock generator (which goes to the AU5426),
> i.e. the "100MHz,3.3V,3225"
> - clock buffer that fans the 100 MHz clock out to the
> PCIe interfaces, i.e. the Aurasemi AU5426
>
> - ROCK 3B:
> - 25 MHz "passive" crystal (which goes to the PI6C557)
> - clock generator that uses the 25 MHz crystal to generate
> 100 MHz and fan it out to the PCIe interfaces, i.e. the
> Diodes PI6C557
>
> (The "passive" 25 MHz crystal is very similar to the main 24 MHz
> crystal used by the SoC, also known as xin24m.)
>
> However, simplifying and abstracting the things out a bit should
> be fine, to end up with the following:
>
> - ROCK 5 ITX:
> - "black box" that generates fixed 100 MHz clocks for the
> PCIe interfaces, which can be gated
>
> - ROCK 3B:
> - "black box" that generates fixed 100 MHz clocks for the
> PCIe interfaces, which can be gated
>
> ... and that's our, hopefully, gated-clock. :)

I don't think we want too many black-boxes. Devicetree's purpose is to
describe the hardware as best as possible, the OS can make of it what it
wants afterwards ;-) .

That was also Stephen's point in v1 when he wanted to distinguish
between the input-less oscillator and the Diodes things that needs an
input clock.

> > The interesting part of the Diodes ICs is that they actually allow
> > configuration of the generated frequency via their S0 + S1 pins,
> > though they might be statically set in the board layout without
> > being user configurable. (Rock3b does it this way for example)
>
> The Aurasemi AU5426 also has a few tricks up its sleeve. :) For
> example, it can also use a "passive" crystal instead of using an
> external clock generator, i.e. it can be configured to work in
> the same "big picture" layout as the PI6C577.
>
> >> - 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 would only require the vdd supply though.
> >
> > On the Rock 5 ITX, the chip does have an enable-gpio input, but that is
> > tied directly to the voltage rail and is not user controllable.
>
> Isn't that voltage rail (VCC3V3_PI6C_05, provided by the U90099
> regulator,
> which is an RT9193) actually enabled by GPIO1_A4_d (see pages 14 and 35
> in the ROCK 5 ITX schematic and follow PCIE30x_PWREN_H)?
>
> Unless I'm missing something, that's the source of all troubles, because
> we basically also need to enable PCIE30x_PWREN_H when only the other M.2
> slot is in use, or when it's enumerated first. Thus, we need
> enable-gpios
> to be able to enable the VCC3V3_PI6C_05 independently.

And you correctly described the problem I faced and that got me on that
journey (sata-pcie-controller probing before M.2 attached controller).

PCIE30x4_PWREN_H actually drives two separate regulators though,
U90098 and U90099 .

U90098 creates VCC3V3_MKEY which is the supply for the main M.2 slot
U90099 creates VCC3V3_PI6C_05 which is the oscillator and Au5426 supply

PCIE30x4_PWREN_H definitly belongs to a regulator node in the devicetree
that then either the M.2 slot or the oscillator can enable first.

> >> - 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
> >
> > "gated-oscillator" perhaps?
>
> But what's the output of an oscillator, which we actually care about?
> It's nothing more but a clock. :)
>
> > This would make it more explicit that we're talking about a root
> > for clock signals. "gated-clock" can be anything, in the middle
> > of a clock tree. Having a very generic name, also invites misuse.
>
> I can't escape wondering why doesn't "fixed-clock", which is also
> a very generic name, invite abuse?

I guess it does - see the misuse in the rk3588 tiger and jaguar boards.
But it is sort of grandfathered I guess. It is still from a time more than
12 years ago, when architectures (like sunxi) tried to model their clock-
controllers via individual interconnected dt-nodes.

> Speaking about the root of clock signals, the above-mentioned xin24m,
> which is represented in DTs as a fixed-clock, is the root of pretty
> much everything. Also, it technically isn't a clock, it's a crystal.
> That's another example of the above-mentioned abstraction, in which
> we care about the way sotware sees it, which is a clock.

At this point I think we should dt-people weigh in more on which
direction to take ;-) .

Because as a lot of dt-binding review in the past mentioned, the
"software-perspective" doesn't matter for the binding.

Heiko

> >> - 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/
> >
> > [8] https://www.diodes.com/assets/Datasheets/PI6C557-03.pdf
> > [9] https://dl.radxa.com/rock3/docs/hw/3b/Radxa_ROCK_3B_V1.51_SCH.pdf
> > page 31, bottom left of the page
> >
> >
> >> >> > +
> >> >> > +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>;
> >> >> > + };
> >> >> > +...
>