Re: Question about fixed-clock

From: Daniel Mack
Date: Tue Feb 19 2013 - 12:32:22 EST

This is a multi-part message in MIME format.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@xxxxxxxxx> 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.