linux-kernel - Re: [PATCH v15 5/6] dt-bindings: clock: meson: add A1 Peripherals clock controller bindings

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Message-ID: <CAFBinCAk9+Km3BssA8d8nc_Z_GbhY87FD3qQRpZ2k7ChKt7TBg@mail.gmail.com>
Date:   Mon, 29 May 2023 22:38:33 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Dmitry Rokosov <ddrokosov@...rdevices.ru>,
        Conor Dooley <conor.dooley@...rochip.com>,
        krzysztof.kozlowski+dt@...aro.org
Cc:     neil.armstrong@...aro.org, jbrunet@...libre.com,
        mturquette@...libre.com, sboyd@...nel.org, robh+dt@...nel.org,
        khilman@...libre.com, jian.hu@...ogic.com, kernel@...rdevices.ru,
        rockosov@...il.com, linux-amlogic@...ts.infradead.org,
        linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v15 5/6] dt-bindings: clock: meson: add A1 Peripherals
 clock controller bindings

Hi Dmitry,

On Mon, May 22, 2023 at 3:00 PM Dmitry Rokosov <ddrokosov@...rdevices.ru> wrote:
[...]
> > This IP block has at least one additional input called "sys_pll_div16".
> > My understanding is that the "sys_pll_div16" clock is generated by the
> > CPU clock controller. Support for the CPU clock controller
> > (dt-bindings and a driver) will be added at a later time by Dmitry.
> > How can we manage incrementally implementing the clock controllers?
> > From a hardware perspective the "sys_pll_div16" input is mandatory.
> > How to manage this in the .dts patches then (for example: does this
> > mean that Dmitry can only add the clock controller to the .dts when
> > all clock controller bindings have been implemented - or is there
> > another way)?
>
> You're absolutely right: currently, not all inputs are supported because
> the CPU clock controller isn't ready yet – I'm working on it at the
> moment.
>
> I understand your concerns about bindings and schema description, but
> there is an issue to be considered. I'm developing the entire clock
> controller A1 subsystem incrementally in three stages: peripherals and
> PLL, CPU, and Audio. This is because the CPU can operate at a static
> frequency and voltage, and the board boots normally without the CPU
> clock controller, thermal sensor, and OPP table. Audio is also
> important, but it's optional. On the other hand, without setting up the
> peripherals and PLL controllers, the board won't function because
> they're fundamental.
I understand your approach and I like it (without that incremental
approach you would probably be looking at a series with 15-20
patches).

Maybe the dt-binding maintainers have a suggestion for us here?
Let me try to summarize the issue in a few bullet points:
- There's (at least) four clock controllers on the Amlogic A1 SoC
- Some of these clock controllers take the outputs of another clock
controller as inputs
- In this series patch the peripheral clock controller has an input
called "sys_pll_div16"
- The clock controller which provides the "sys_pll_div16" clock is not
implemented yet (my understanding is that implementing it and adding
it to this series is not easy: it would add even more patches that
need to be reviewed and in general it's a tricky clock controller to
implement as it manages the CPU clocks)

> Right now, we're in the first stage of the plan. Unfortunately, I can't
> disclose the exact names and number of clock bindings for the CPU and
> Audio, as they're still in development and only exist in my head or
> draft versions.
>
> If possible, I'd prefer to provide the new bindings and connections once
> all the appropriate drivers are finalized.
Question to Conor and Krzysztof (assuming you read my summary above):
Is it fine that Dmitry adds additional inputs to the peripheral clock
controller binding in later patches?
If not: how can we proceed in case we need to add them now (the
dt-binding example is the easy part for me as we can just make up a
phandle like &sys_pll_div16_clk and use that - but this can't work
when Dmitry tries to add the clock controller to meson-a1.dtsi)

PS: Dmitry is trying to get this series into Linux 6.5. As far as I
remember the common clock maintainers don't take pull requests with
new features after -rc6 (which is in less than two weeks).
So time is getting a bit short and for me this is the very last
outstanding question. If you say that it's fine to add clocks later on
this will immediately get my Reviewed-by.


Best regards,
Martin