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

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Message-ID: <CACxGe6sh+ofJWSA8xpDazEKVOUOpaHK45Qm8C20zpyRaLT77Mg@mail.gmail.com>
Date:	Tue, 13 Dec 2011 11:03:57 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Rob Herring <robherring2@...il.com>
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

On Tue, Dec 13, 2011 at 11:01 AM, Rob Herring <robherring2@...il.com> wrote:
> 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).

No.  The #clock-cells does not include the phandle.  Never did.

g.
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