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

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Date:	Tue, 17 Sep 2013 06:08:48 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org, Mike Turquette <mturquette@...aro.org>,
	Hyun Kwon <hyunk@...inx.com>, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rob Herring <rob.herring@...xeda.com>,
	linux-arm-kernel@...ts.infradead.org,
	Rob Landley <rob@...dley.net>,
	Grant Likely <grant.likely@...aro.org>,
	Soren Brinkmann <soren.brinkmann@...inx.com>
Subject: Re: [PATCH] clk: si570: Add a driver for SI570 oscillators

On 09/17/2013 12:59 AM, Sebastian Hesselbarth wrote:
> On 09/16/2013 08:37 PM, Stephen Warren wrote:
>> 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.
>
> For the clk-si5351 programmable clock driver in mainline, it already
> uses "clock-frequency" for initial clock setup but allows to set it
> later on. IMHO that is ok, because from a initial point-of-view, an
> initial frequency is fixed. As soon as the driver takes over, the user
> is free to do whatever he wants and should not be limited by DT.
>
> But if we vote against that approach, we should probably also modifiy
> clk-si5351 accordingly.
>

Not me; I am fine either way. Howeber, if there is a use case requiring both
it should be permitted, and if you ask me to vote I'll vote for being permissive.

Guenter


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