Re: [RFC v2 4/9] of: add clock providers

[Grant Likely]

On Mon, Dec 12, 2011 at 4:29 PM, Jamie Iles <jamie@xxxxxxxxxxxxx> wrote:
> Hi Grant,
>
> I'm still going through these and trying to digest them but a couple of
> quick questions/comments.
>
> Jamie
>
> On Mon, Dec 12, 2011 at 03:02:04PM -0700, Grant Likely wrote:
>> diff --git a/Documentation/devicetree/bindings/clock/clock-bindings.txt b/Documentation/devicetree/bindings/clock/clock-bindings.txt
>> new file mode 100644
>> index 0000000..e40c436
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/clock/clock-bindings.txt
>> @@ -0,0 +1,114 @@
>> +This binding is a work-in-progress, and are based on some experimental
>> +work by benh[1].
>> +
>> +Sources of clock signal can be represented by any node in the device
>> +tree.  Those nodes are designated as clock providers.  Clock consumer
>> +nodes use a phandle and clock specifier pair to connect clock provider
>> +outputs to clock inputs.  Similar to the gpio specifiers, a clock
>> +specifier is an array of one more more cells identifying the clock
>> +output on a device.  The length of a clock specifier is defined by the
>> +value of a #clock-cells property in the clock provider node.
>> +
>> +[1] http://patchwork.ozlabs.org/patch/31551/
>> +
>> +==Clock providers==
>> +
>> +Required properties:
>> +#clock-cells:           Number of cells in a clock specifier; typically will be
>> +                set to 1
>
> I'm not sure I fully understand what the extra cells actually mean for
> clocks.  I think the first integer is the clock output to use but some
> of the versatile and highbank ones only have a phandle or is it more
> implementation defined?  The clock-output-names description hints at
> recommended, so I find this a little confusing, but that could just be
> me!

I'm following convention here that has been established with
interrupts, gpios, and others.  Sometimes more information is needed
that just the clock number.  Using #clock-cells gives a clock provider
the option of having additional fields for clock flags or other data.
This is very much implementation defined.  Simple clock providers that
only have a single clock output can easily use #clock-cells = <0>.
Providers with multiple outputs will need to use 1 or more cells.

>> +Optional properties:
>> +clock-output-names: Recommended to be a list of strings of clock output signal
>> +                 names indexed by the first cell in the clock specifier.
>> +                 However, the meaning of clock-output-name is domain
>> +                 specific to the clock provider, and is only provided to
>> +                 encourage using the same meaning for the majority of clock
>> +                 providers.  This format may not work for clock providers
>> +                 using a complex clock specifier format.  In those cases it
>> +                 is recommended to omit this property and create a binding
>> +                 specific names property.
>> +
>> +                Clock consumer nodes must never directly reference
>> +                the provider's clock-output-name property.
>> +
>> +For example:
>> +
>> +    oscillator {
>> +        #clock-cells = <1>;
>> +        clock-output-names = "ckil", "ckih";
>> +    };
>> +
>> +- this node defines a device with two clock outputs, the first named
>> +  "ckil" and the second named "ckih".  Consumer nodes always reference
>> +  clocks by index. The names should reflect the clock output signal
>> +  names for the device.
>> +
>> +==Clock consumers==
>> +
>> +Required properties:
>> +clocks:              List of phandle and clock specifier pairs, one pair
>> +             for each clock input to the device.
>
> Some of the highbank and versatile devicetree nodes have clocks
> properties that aren't a pair e.g. versatile timer has
> "clocks = <&tim_clk>;".

It's still a pair.... it's just that the specifier portion has a zero
length.  :-)  I do agree that the documentation needs work though.

>
>> +clock-names: List of clock input name strings sorted in the same
>> +             order as the clocks property.  Consumers drivers
>> +             will use clock-names to match clock input names
>> +             with clocks specifiers.
>
> The versatile and highbank patches appears to omit this required
> property in several nodes.  So is this really optional?

You're right, it's not required.  I'll move it to optional.

g.
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