Re: [PATCH] clk: mediatek: Export CPU mux clocks for CPU frequency control

[Pi-Cheng Chen <pi-cheng.chen@linaro.org>]

On 5 March 2015 at 15:42, Sascha Hauer <s.hauer@pengutronix.de> wrote:
>
> +Cc Viresh Kumar
>
> Viresh, this is the patch for the underlying clocks for the Mediatek
> cpufreq driver.
>
> On Thu, Mar 05, 2015 at 10:43:21AM +0800, Pi-Cheng Chen wrote:
>> Hi Sascha,
>>
>> On 4 March 2015 at 19:21, Sascha Hauer <s.hauer@pengutronix.de> wrote:
>> > On Wed, Mar 04, 2015 at 06:49:11PM +0800, pi-cheng.chen wrote:
>> >> This patch adds CPU mux clocks which are used by Mediatek cpufreq driver
>> >> for intermediate clock source switching. This patch is based on Mediatek
>> >> clock driver patches[1].
>> >>
>> >> [1] http://thread.gmane.org/gmane.linux.kernel/1892436
>> >>
>> >> Signed-off-by: pi-cheng.chen <pi-cheng.chen@linaro.org>
>> >> ---
>> >> +static long clk_cpumux_determine_rate(struct clk_hw *hw, unsigned long rate,
>> >> +                                   unsigned long min_rate,
>> >> +                                   unsigned long max_rate,
>> >> +                                   unsigned long *best_parent_rate,
>> >> +                                   struct clk_hw **best_parent_p)
>> >> +{
>> >> +     struct clk *clk = hw->clk, *parent;
>> >> +     unsigned long parent_rate;
>> >> +     int i;
>> >> +
>> >> +     for (i = MAINPLL_INDEX; i >= ARMPLL_INDEX; i--) {
>> >> +             parent = clk_get_parent_by_index(clk, i);
>> >> +             if (!parent)
>> >> +                     return 0;
>> >> +
>> >> +             if (i == MAINPLL_INDEX) {
>> >> +                     parent_rate = __clk_get_rate(parent);
>> >> +                     if (parent_rate == rate)
>> >> +                             break;
>> >> +             }
>> >> +
>> >> +             parent_rate = __clk_round_rate(parent, rate);
>> >> +     }
>> >> +
>> >> +     *best_parent_rate = parent_rate;
>> >> +     *best_parent_p = __clk_get_hw(parent);
>> >> +     return parent_rate;
>> >> +}
>> >
>> > Why this determine_rate hook? If you want to switch the clock to some
>> > intermediate parent I would assume you do this explicitly by setting the
>> > parent and not implicitly by setting a rate.
>> >
>>
>> I use determine_rate hook here because I am using generic cpufreq-dt
>> driver and I want to make clock switching transparent to cpufreq-dt.
>> I.e. when I am trying to switch the clock from ARMPLL to MAINPLL, I
>> call clk_set_rate() with the rate of MAINPLL, and determine_rate will
>> select MAINPLL as the new parent.
>
> We have clk_set_parent for changing the parent and clk_set_rate to
> change the rate. Use the former for changing the parent and the latter
> for changing the rate. What you are interested in is changing the
> parent, so use clk_set_parent for this and not abuse a side effect
> of clk_set_rate.
>
> My suggestion is to take another approach. Implement clk_set_rate for
> these muxes and in the set_rate hook:
>
> - switch mux to intermediate PLL parent
> - call clk_set_rate() for the real parent PLL
> - switch mux back to real parent PLL

Hi Sascha,

Thanks for your suggestion. I've tried to take this approach, but there's some
issues here.

Calling clk_set_rate() inside the set_rate callback of cpumux will cause
an infinite recursive calling in the clock framework:
mux.set_rate() -> pll.set_rate() -> mux.set_rate -> ...

I've also tries to update pll register settings in the set_rate()
callback of cpumux,
but the PLL clock information will not be correctly updated in this case.

How do you think to create a new "CPU PLL" type of clock and do underlying
mux switching in the set_rate callback of "CPU PLL"?

Best Regards,
Pi-Cheng

>
> This way the things happening behind the scenes are completely transparent
> to the cpufreq driver and you can use cpufreq-dt as is without changes.
>
> Sascha
>
>
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