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

[Vincent Guittot <vincent.guittot-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On 3 July 2014 09:10, Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> wrote:
> On Mon, Jun 30, 2014 at 05:06:16PM +0200, Vincent Guittot wrote:
>> On 30 June 2014 16:58, Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org> wrote:
>> > 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.
>>
>> yes, you're right
>>
>> >
>> >> 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?
>>
>> yes, but  clk_round_rate(clk, ULONG_MAX) will return the max frequency
>> and not the current one. But as you mentioned, it doesn't ensure that
>> it's a possible frequency for the core
>
> Is this an Acked-by ?

 I still think that using the current rate with clk_get_rate is not a
good solution.

Could you give us more details about why you nee to use current clock ?

AFAICT, your patch only makes sense for HMP system which don't have
DVFS, is it your case ?

Vincent

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