linux-kernel - Re: [PATCH] clk: si570: Add a driver for SI570 oscillators

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Message-ID: <52374FF7.8040604@wwwdotorg.org>
Date:	Mon, 16 Sep 2013 12:37:43 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Guenter Roeck <linux@...ck-us.net>
CC:	Soren Brinkmann <soren.brinkmann@...inx.com>,
	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Rob Landley <rob@...dley.net>,
	Mike Turquette <mturquette@...aro.org>,
	Grant Likely <grant.likely@...aro.org>,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Hyun Kwon <hyunk@...inx.com>
Subject: Re: [PATCH] clk: si570: Add a driver for SI570 oscillators

On 09/16/2013 11:35 AM, Guenter Roeck wrote:
> On Mon, Sep 16, 2013 at 10:59:58AM -0600, Stephen Warren wrote:
>> On 09/16/2013 10:49 AM, Guenter Roeck wrote:
>>> On Mon, Sep 16, 2013 at 10:34:28AM -0600, Stephen Warren wrote:
>>>> On 09/12/2013 06:55 PM, Soren Brinkmann wrote:
>>>>> Add a driver for SILabs 570, 571, 598, 599 programmable oscillators.
>>>>> The devices generate low-jitter clock signals and are reprogrammable via
>>>>> an I2C interface.
...
>>>>> +Optional properties:
>>>>> + - initial-fout: Initial output frequency to set during probe
>>>>
>>>> "probe" is a Linux-specific concept. This property should be removed. If
>>>> the driver is asked to set a specific frequency, it should do so, but I
>>>> don't think it should program something pro-actively just because it
>>>> starts up.
>>>>
>>>> If this property is acceptable, it'd be better to describe it more along
>>>> the lines of the following:
>>>>
>>>> initial-fout: The frequency at which the system requires the clock to
>>>> operate.
>>>
>>> It should probably be something like "clock-frequency". In many use cases
>>> the programmed frequency is set to a constant frequency at system startup
>>> and never changed, similar to other clocks.
>>
>> I was going to suggest that too, but re-considered since I think
>> clock-frequency is more appropriate for fixed-frequency clocks, rather
>> than to specify the value at which a programmable clock generator should
>> operate?
>>
>> I don't think we have a good story yet for how to represent
>> how-we-want-the-clock-tree-configured, as opposed to representing the HW
>> itself (which is what DT should be more about).
>
> In many cases the chip _is_ used to generate a fixed frequency, so we will
> have to have a means to describe it. That it _can_ be used differently is a
> different matter. After all, that is true for many clock generators.

Perhaps if clock-frequency is specified, the driver should refuse to
provide anything else. If clock-frequency isn't specified, the driver
shouldn't touch the HW when it initializes, but should honor any
requests that come in from other drivers? That would maintain what I
feel is clock-frequency's connection to being a fixed clock.
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