linux-kernel - Re: [PATCH] cpufreq: imx: Add cpufreq driver for imx7D/Solo SOC

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Date:	Tue, 24 Nov 2015 10:08:17 +0100
From:	Lucas Stach <l.stach@...gutronix.de>
To:	Bai Ping <b51503@...escale.com>
Cc:	rjw@...ysocki.net, viresh.kumar@...aro.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpufreq: imx: Add cpufreq driver for imx7D/Solo SOC

Am Dienstag, den 24.11.2015, 15:35 +0800 schrieb Bai Ping:
> 
> On 2015/11/23 17:40, Lucas Stach wrote:
> > Am Montag, den 23.11.2015, 22:07 +0800 schrieb Bai Ping:
> >> The i.MX7Dual/Solo is a new series of the i.MX SOC family.
> >> The existing cpufreq driver for 'i.MX6' or 'cpufreq-dt' can
> >> NOT match the requirement of this new SOC. This patch adds the
> >> cpufreq driver for i.MX7Dual/Solo.
> >>
> > So, what are those requirements, which could not be matched with
> > cpufreq-dt? We should really try to not add another cpufreq driver.
> the requirement is the PLL1 used to source the CPU core clock can NOT 
> change frequency on the fly,
> during the PLL1 frequency change, not clock output from this PLL1 in a 
> short time. this will lead to glitch
> to the core clock. so before we change the PLL1's frequency, we must 
> switch the CPU core clock to another
> clock source,  after the PLL1 frequency has been changed, we switch back 
> core clock to PLL1.
> > I don't see anything special in here. A single regulator and some clocks
> > needing to be controlled in the right way. That's already handled for
> > i.MX5 with cpufreq-dt. Please look up how it is done there and try to do
> > it the same way for MX7, or provide substantial information why it
> > couldn't be done.
> I have checked the i.MX5 cpufreq, As on i.MX5, no need to take care of 
> the PLL's frequency change flow,
> so the cpufreq-dt is the best one to support cpufreq. But on i.MX7, the 
> PLL design is not the same as on i.MX5,
> additional steps needed in CPU frequency changing flow. the issue that 
> can NOT be tackled by cpufreq-dt is
> additional step used by PLL frequency change.

This is wrong, we have the step clock requirement on MX5 as well, that's
why I asked you to look into this. The MX5 clock controller provides a
virtual ARM clock, that implements the proper clock switch flow. This
allows to reuse cpufreq-dt, thus reducing code duplication in the
cpufreq drivers greatly.

Regards,
Lucas

-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |

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