linux-kernel - Re: Question about fixed-clock

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Message-ID: <5123B719.2080300@gmail.com>
Date:	Tue, 19 Feb 2013 18:32:09 +0100
From:	Daniel Mack <zonque@...il.com>
To:	Mike Turquette <mturquette@...aro.org>
CC:	Fabio Estevam <festevam@...il.com>,
	Russell King <linux@....linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Afzal Mohammed <x0148406@...com>, paul@...an.com, rnayak@...com
Subject: Re: Question about fixed-clock

On 19.02.2013 18:22, Mike Turquette wrote:
> Quoting Daniel Mack (2013-02-19 01:53:18)
>> Hi Fabio,
>> Hi Afzal,
>>
>> On 19.02.2013 02:33, Fabio Estevam wrote:
>>> On Mon, Feb 18, 2013 at 8:55 PM, Daniel Mack <zonque@...il.com> wrote:
>>>> Hi,
>>>>
>>>> This might be a stupid question, but I'm somehow stuck here. I'm using a
>>>> driver with the following DTS sub-node:
>>>>
>>>>         ref25: ref25M {
>>>>                 compatible = "fixed-clock";
>>>>                 #clock-cells = <0>;
>>>>                 clock-frequency = <25000000>;
>>>>         };
>>>>
>>>>         clock-generator@0 {
>>>>                                 /* ... */
>>>>                                 #clock-cells = <1>;
>>>>                                 clocks = <&ref25>;
>>>>         }
>>>>
>>>> The device driver for clock-generator uses something like the following
>>>> call to get its clock:
>>>>
>>>>         clk = of_clk_get(np, 0);
>>>>
>>>> but the return value is ERR_PTR(-ENOENT) and I also can't find this
>>>> clock in the clk debugfs tree.
>>>>
>>>> This is on a OMAP/AM33xx device with kernel 3.8-rc7 plus the -next tips
>>>> of arm-soc and omap, but with no other special clock options selected in
>>>> the config. Is there anything I'm missing to correctly instantiate the
>>>> dummy clock?
>>>
>>> Have you registered it with clk_register_fixed_rate() ?
>>>
>>> In imx we use imx_clk_fixed, which in turns call  clk_register_fixed_rate().
>>>
>>> Take a look at arch/arm/mach-imx/clk-imx51-imx53.c for a reference.
>>
>> Hmm no, I didn't do anything else than adding it to the DT in the first
>> place, hoping that a driver will pick it up and add the clock for me.
>> But it turns out that of_clk_init() is not called at all on my platform.
>>
>> I'm doing this now from omap_generic_init() and it works. If that's an
>> appropriate place to call it, I can provide a patch.
>>
> 
> You can provide a patch, but your example above is for a dummy clock,
> correct?

No, it's a real clock with a fixed frequency on the board, which feeds
another clock chip. I thought this is what fixed-clock is for, just like
what a fixed-regulator does? Or how would you describe such a thing in
DT, in order to use it as input to another device?

> It would be best to post your patch along with a real-world
> use for calling of_clk_init().

Not sure what you mean exactly, but the patch is attached. However, I
still think I lack some understanding here - the fact that compatible
strings have to be passed explicitly from generic code feels wrong.


Thanks,
Daniel


View attachment "0001-ARM-OMAP-generic-add-call-to-of_clk_init.patch" of type "text/x-patch" (1337 bytes)