Re: [PATCH v1 2/2] cpufreq: tegra20: Use PLL_C as intermediate clock source

[Peter De Schrijver]

On Thu, May 24, 2018 at 03:49:22PM +0300, Dmitry Osipenko wrote:
> On 24.05.2018 13:04, Peter De Schrijver wrote:
> > On Wed, May 23, 2018 at 07:00:20PM +0300, Dmitry Osipenko wrote:
> >> PLL_C is running at 600MHz which is significantly higher than the 216MHz
> >> of the PLL_P and it is known that PLL_C is always-ON because AHB BUS is
> >> running on that PLL. Let's use PLL_C as intermediate clock source, making
> >> CPU snappier a tad during of the frequency transition.
> >>
> > 
> > pll_c isn't necessarily 600Mhz when used as a source for the second display
> > head.
> 
> Hmm, indeed.
> 
> Even if PLL_C rate will be adjusted, it will be higher than the PLL_P.. won't
> it? That's likely to be good enough.
> 

Yes. I think it can be always higher than pll_p, but that assumes the display
driver will always program the highest possible rate for pll_c for a given mode
and then program the display divider to divide it down to the required rate.

> Do you know if any of the available CCLK parents has a glitch-less rate
> switching? I.e. CPU won't hang on the rate switch.
> 

Tegra20 doesn't have dynamic ramp PLLs no. So you always have to switch to a
backup clock source before changing the rate of pll_x.

> There is other possible 600MHz source, the PLL_M. Can we use it? This one also
> may become dynamic if we'll consider implementing the memory scaling, but the
> memory frequency probably will fit the transition role pretty well.

I think this should work, but as you mention it may very well be lower than
pll_p if Tegra20 EMC scaling is re-introduced. I think that's why historically
this was never done. 

Peter.