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

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Date:   Mon, 5 Jun 2023 12:13:05 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Mike Looijmans <mike.looijmans@...ic.nl>,
        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 12:01, 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...

Best regards,
Krzysztof