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    SubjectRe: [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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