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

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Message-ID: <33119212.aRNtrjHk3s@diego>
Date: Sat, 27 Jul 2024 13:25:31 +0200
From: Heiko Stübner <heiko@...ech.de>
To: Conor Dooley <conor@...nel.org>, Stephen Boyd <sboyd@...nel.org>
Cc: mturquette@...libre.com, robh@...nel.org, krzk+dt@...nel.org,
 conor+dt@...nel.org, linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-rockchip@...ts.infradead.org
Subject:
 Re: [PATCH v2 1/3] dt-bindings: clocks: add binding for
 voltage-controlled-oscillators

Am Samstag, 27. Juli 2024, 00:21:44 CEST schrieb Stephen Boyd:
> Quoting Conor Dooley (2024-07-18 08:59:50)
> > 
> > FWIW, I wouldn't classify this as device-specific. "enable-gpios" and
> > "vdd-supply" are pretty generic and I think the latter is missing from
> > the vast majority of real* "fixed-clocks". I would expect that devices
> > where the datasheet would call
> > 
> > * Real because there's plenty of "fixed-clocks" (both in and out of tree)
> > that are used to work around the lack of a clock-controller driver for an
> > SoC.
> 
> I agree!
> 
> > 
> > > 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]
> > 
> > I think that that is something that could be changed, it's "just" a
> > comment in some code! Sounds like Stephen disagrees though :)
> 
> It's more about making a clear break from the fixed-clock binding so
> that the extra properties are required.
> 
> > 
> > > - Stephen also suggested a separate binding [2]
> > 
> > I liked your "gated-oscillator" suggestion in another reply, but
> > "gated-fixed-clock" might be a better "thematic" fit since this is a
> > special case of fixed-clocks?
> > 
> 
> It looks to me like we've arrived at the hardest problem in computer
> science, i.e. naming. Any of these names is fine. I'd look to see what
> those parts on mouser are called and use that to drive the compatible
> name decision if you can't decide. The description section in the
> binding could be verbose and link to some parts/pdfs if that helps too.
> In the past I've seen EEs call these things clock buffers. I'm not a
> classically trained EE myself but it usually helps to use similar names
> from the schematic in DT because DT authors are sorta translating
> schematics to DT.

TL;DR: I'm fine with both "gated-oscillator" or "gated-fixed-clock" .
Some tiny part in the back of my head wants to name things in the most
specific way aka "gated-oscillator", but I guess "gated-fixed-clock" will
possibly spare us the naming dance in the future :-)

So I guess if nobody objects anymore, I'll go with "gated-fixed-clock".

--------- 8< --------
Some background stuff for the oscillator / clock-buffer difference,
which are actually both used on the Rock 5 ITX in question:

[my ASCII-art may not survive mail readers]
                                        ------------
VCC3v3_PI6C (to both VDD + Enable) -----| VCC*     | - CLKoutA - to PCIe
  |                                     |          |
--------------------                    |          | - CLKoutB - to PCIe
| 100MHz,3.3V,3225 |-------XTAL_IN_OUT -| Au5426   |
--------------------                    |          | - REFout (unconnected)
                                        ------------

Just asking Google for that "100MHz,3.3V,3225" brings me to
"100 MHz Standard Clock Oscillators" on Mouser.

The Au5426 from Aurasemi is a "4 Differential and 1 LVCMOS Output Ultra
Low Jitter High Performance Buffer" - aka a clock-buffer.

In the Rock-5-ITX patch, I opted to ignore it, because on _that_ board
it is transparent to the system, enabled by the same sources as the
crystal and statically configured.

On the other hand, the Au5426 actually _has_ input pins to select
its working mode: 
- select between different clock sources
- enable/disable the output of the input clock as refclk
- configure the clock buffer type (lvpecl, lvds, hcsl, hiz)

So I didn't want to conjure a binding for that stuff out of thin air :-)


Heiko