linux-kernel - Re: [PATCH v2 1/2] dt-bindings: clock: Add nvmem-clock

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Date:   Mon, 5 Jun 2023 13:56:24 +0200
From:   Mike Looijmans <mike.looijmans@...ic.nl>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        devicetree@...r.kernel.org, linux-clk@...r.kernel.org
CC:     Conor Dooley <conor+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: clock: Add nvmem-clock

On 05-06-2023 12:13, Krzysztof Kozlowski wrote:
> On 05/06/2023 12:01, Mike Looijmans wrote:
>> On 31-05-2023 21:27, Krzysztof Kozlowski wrote:
>>> On 26/05/2023 16:38, Mike Looijmans wrote:
>>>> Add bindings for a fixed-rate clock that retrieves its rate from an
>>>> NVMEM provider. This allows to store clock settings in EEPROM or EFUSE
>>>> or similar device.
>>>>
>>>> Component shortages lead to boards being shipped with different clock
>>>> crystals, based on what was available at the time. The clock frequency
>>>> was written to EEPROM at production time. Systems can adapt to a wide
>>>> range of input frequencies using the clock framework, but this required
>>>> us to patch the devicetree at runtime or use some custom driver. This
>>>> provides a more generic solution.
>>> This does not look like real hardware. I mean, the clock does not fetch
>>> its rate from nvmem, right? It's the Linux which does it, so basically
>>> you described here driver, not hardware.
>> Right, this just reads a setting from an NVMEM provider.
>>> Extend existing fixed-clock bindings to allow reading frequency via
>>> nvmem cells.
>> I just tried and implemented this, but it does not work. The reason is
>> that the fixed-clock implementation returns "void" in its
>> of_fixed_clk_setup() init function. The nvmem provider returns
>> EPROBE_DEFER because it isn't ready at this early stage, and this error
>> will not be propagated up because of the "void" signature. Thus, it's
>> never retried and the clock just disappears.
> Linux driver problems are not a reason to add bindings for virtual
> hardware...
>
Okay, so what are you proposing?

The implementation needs a compatible string other than "fixed-clock", 
because of the way the fixed-clock driver is being loaded (it can never 
be deferred).

So I should add another compatible string to fixed-clock.yaml? That'd 
make the syntax in fixed-clock.yaml rather awkward, since some 
properties would only apply to some compatible strings?

-- 
Mike Looijmans
System Expert

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