Re: [PATCH v2 1/2] ARM: topology: Use a clock if possible to get the CPU frequency

[Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

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On Mon, Jun 30, 2014 at 04:48:35PM +0200, Vincent Guittot wrote:
> On 30 June 2014 16:01, Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> wrote:
> > On Mon, Jun 30, 2014 at 03:27:21PM +0200, Vincent Guittot wrote:
> >> >> >> -             rate = of_get_property(cn, "clock-frequency", &len);
> >> >> >> -             if (!rate || len != 4) {
> >> >> >> -                     pr_err("%s missing clock-frequency property\n",
> >> >> >> -                             cn->full_name);
> >> >> >> +             clk = of_clk_get(cn, 0);
> >> >> >> +             if (!IS_ERR(clk))
> >> >> >> +                     rate = clk_get_rate(clk);
> >> >>
> >> >> We need the max frequency as it will be used to weight the different
> >> >> CPUs capacity. How do you ensure that the current clock rate is the
> >> >> max one ?
> >> >
> >> > Hmm, the clock-frequency attribute in the ePAPR is defined at the
> >> > current CPU frequency, not the max one.
> >>
> >> What means current frequency in device tree when DVFS is involved ?
> >
> > The ePAPR states that clock-frequency is supposed to be "the current
> > clock speed of the CPU in Hertz". It's exactly what my patch add.
> >
> > Now, you're right, DVFS would be an issue here with clock-frequency,
> > but this patch actually makes it easier to deal with, since you only
> > get a reference to a clock, and you can get its rate at any given
> > time.
> 
> and what about using clk_round_rate(clk, ULONG_MAX) ?

Well, the clock itself might generate a frequency higher that what the
CPU can run at, so I'm not sure it's a valid assumption.

> We will not be dependent of when we parse DT

You lost me there. clk_round_rate and clk_get_rate are available at
the same time in the boot process, aren't they?

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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