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

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Message-ID: <4EE792DF.1010701@gmail.com>
Date:	Tue, 13 Dec 2011 12:01:03 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Grant Likely <grant.likely@...retlab.ca>
CC:	Jamie Iles <jamie@...ieiles.com>, linux-kernel@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org,
	Sascha Hauer <kernel@...gutronix.de>,
	Mike Turquette <mturquette@...com>
Subject: Re: [RFC v2 4/9] of: add clock providers

Grant,

On 12/13/2011 11:54 AM, Grant Likely wrote:
> On Mon, Dec 12, 2011 at 4:29 PM, Jamie Iles <jamie@...ieiles.com> 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.
> 

Aren't you off by 1 here? The minimum cells is 1 to hold the phandle of
the source/parent. Multiple outputs will need a cell size of 2 (typically).

Rob
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