Re: [PATCH v2 1/7] cpufreq: cpufreq-cpu0: allow optional safe voltage during frequency transitions

Thomas Abraham
2014-02-03T16:06:23Z

On Thu, Jan 30, 2014 at 8:39 PM, Heiko Stübner <heiko@sntech.de> wrote:
> On Thursday, 30. January 2014 18:23:44 Thomas Abraham wrote:
>> Hi Mike,
>>
>> On Wed, Jan 29, 2014 at 12:17 AM, Mike Turquette <mturquette@linaro.org>
> wrote:
>> > On Mon, Jan 27, 2014 at 9:30 PM, Thomas Abraham <ta.omasab@gmail.com>
> wrote:
>> >> Hi Mike,
>> >>
>> >> On Tue, Jan 28, 2014 at 1:55 AM, Mike Turquette <mturquette@linaro.org>
> wrote:
>> >>> Quoting Thomas Abraham (2014-01-18 04:10:51)
>> >>>
>> > As far as I can tell
>> > the remux does not happen because it is necessary to generate the
>> > required clock rate, but because we don't want to run the ARM core out
>> > of spec for a short time while the PLL relocks. Assuming I have that
>> > part of it right, I prefer for the parent mux operation to be a part
>> > of the CPUfreq driver's .target callback instead of hidden away in the
>> > clock driver.
>>
>> The re-parenting is mostly done to keep the ARM CPU clocked while the
>> PLL is stopped, reprogrammed and restarted. One of the side effects of
>> that is, the clock speed of the temporary parent could be higher then
>> what is allowed due to the ARM voltage at the time of re-parenting.
>> That is the reason to use the safe voltage.
>
> The Rockchip-SoCs use something similar, so I'm following quite closely what
> Thomas is trying to do here, as similar solution would also solve this issue
> for me.
>
> On some Rockchip-SoCs even stuff like pclk and hclk seems to be sourced from
> the divided armclk, creating additional constraints.
>
> But on the RKs (at least in the upstream sources) the armclk is simply equal
> to the pll output. A divider exists, but is only used to make sure that the
> armclk stays below the original rate when sourced from the temp-parent, like
>
> if (clk_get_rate(temp_parent) > clk_get_rate(main_parent)
> set_divider(something so that rate(temp) <= rate(main)
> clk_set_parent(...)
>
> Isn't there a similar possiblity on your platform, as it would remove the need
> for the safe-voltage?

Hi Heiko,

Yes, this works too! I have tested this method on Exynos4210,
Exynos4412 and Exynos5250 and it works fine without any need for safe
voltage. This is much better than using safe voltage. Thank you for
suggesting this.

Regards,
Thomas.

>
>
> In general I also like the approach of hiding the rate-change logic inside
> this composite clock, as the depending clocks can be easily kept in sync. As
> with the Rockchips the depending clocks are different for each of the three
> Cortex-A9 SoCs I looked at, it would be "interesting" if all of this would
> need to be done in a cpufreq driver.
>
>
> Heiko
>