Hi Peter,
On Tue, Apr 25, 2017 at 1:02 PM, Peter De Schrijver
<pdeschrijver@xxxxxxxxxx> wrote:
> On Tue, Apr 25, 2017 at 12:28:29PM +0200, Geert Uytterhoeven wrote:
>> On Tue, Apr 25, 2017 at 12:18 PM, Peter De Schrijver
>> <pdeschrijver@xxxxxxxxxx> wrote:
>> > On Mon, Apr 24, 2017 at 10:09:10AM +0200, Geert Uytterhoeven wrote:
>> >> On Mon, Apr 24, 2017 at 10:03 AM, Peter De Schrijver
>> >> <pdeschrijver@xxxxxxxxxx> wrote:
>> >> >> > --- a/drivers/clk/renesas/rcar-gen3-cpg.c
>> >> >> > +++ b/drivers/clk/renesas/rcar-gen3-cpg.c
>> >> >>
>> >> >> > +static unsigned long cpg_z_clk_recalc_rate(struct clk_hw *hw,
>> >> >> > + unsigned long parent_rate)
>> >> >> > +{
>> >> >> > + struct cpg_z_clk *zclk = to_z_clk(hw);
>> >> >> > + unsigned int mult;
>> >> >> > + unsigned int val;
>> >> >> > + unsigned long rate;
>> >> >> > +
>> >> >> > + val = (clk_readl(zclk->reg) & CPG_FRQCRC_ZFC_MASK)
>> >> >> > + >> CPG_FRQCRC_ZFC_SHIFT;
>> >> >> > + mult = 32 - val;
>> >> >> > +
>> >> >> > + rate = div_u64((u64)parent_rate * mult + 16, 32);
>> >> >> > + /* Round to closest value at 100MHz unit */
>> >> >> > + rate = 100000000*DIV_ROUND_CLOSEST(rate, 100000000);
>> >> >>
>> >> >> Mike, Stephen: what's your opinion about such rounding tricks?
>> >> >
>> >> > Is this an actual divider or a pulse skipper?
>> >>
>> >> Forgive my ignorance, but what is the difference?
>> >
>> > A pulse skipper, as the name says, skips pulses. Eg, if you configure the
>> > skipper to 3/4, it will skip 1 input clock pulse out of 4. The pulse width
>> > will be the same as the parent clock pulse width though. A divider stretches
>> > the clock pulse. So the output of a divider configured to 1/2 will have a
>> > pulse width which is twice as large as the pulse width of the input clock.
>>
>> OK, thanks! So a pulse skipper changes the duty cycle of the signal.
>
> Yes, it does.
>
>> I would expect this to be a real divider, as that would allow the Cortex A53
>> core to do work on both edges of the clock signal.
>
> Maybe. Although I know we've run A53s from skippers. One difference is that a
> real divider allows you to lower the voltage, while a skipper does not,
> because it only changes the duty cycle as you mentioned.
Then it must be a real divider, as it supports DVFS.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
