linux-kernel - Re: [PATCH V5 1/2] cpufreq: add support for intermediate (stable) frequencies

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Message-ID: <CAD=FV=W3gOMaJ3gFCA1p4-Ee8f5b+ntvgfmMX5Zj62-Wu9htzQ@mail.gmail.com>
Date:	Thu, 5 Jun 2014 12:51:30 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arvind Chauhan <arvind.chauhan@....com>,
	Stephen Warren <swarren@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Thomas Abraham <thomas.abraham@...aro.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>
Subject: Re: [PATCH V5 1/2] cpufreq: add support for intermediate (stable) frequencies

Viresh,

On Mon, Jun 2, 2014 at 10:19 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Douglas Anderson, recently pointed out an interesting problem due to which
> udelay() was expiring earlier than it should.
>
> While transitioning between frequencies few platforms may temporarily switch to
> a stable frequency, waiting for the main PLL to stabilize.
>
> For example: When we transition between very low frequencies on exynos, like
> between 200MHz and 300MHz, we may temporarily switch to a PLL running at 800MHz.
> No CPUFREQ notification is sent for that. That means there's a period of time
> when we're running at 800MHz but loops_per_jiffy is calibrated at between 200MHz
> and 300MHz. And so udelay behaves badly.
>
> To get this fixed in a generic way, lets introduce another set of callbacks
> get_intermediate() and target_intermediate(), only for drivers with
> target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
>
> get_intermediate() should return a stable intermediate frequency platform wants
> to switch to, and target_intermediate() should set CPU to to that frequency,
> before jumping to the frequency corresponding to 'index'. Core will take care of
> sending notifications and driver doesn't have to handle them in
> target_intermediate() or target_index().
>
> NOTE: ->target_index() should restore to policy->restore_freq in case of
> failures as core would send notifications for that.
>
> Tested-by: Stephen Warren <swarren@...dia.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  Documentation/cpu-freq/cpu-drivers.txt | 29 ++++++++++++++-
>  drivers/cpufreq/cpufreq.c              | 67 ++++++++++++++++++++++++++++++----
>  include/linux/cpufreq.h                | 25 +++++++++++++
>  3 files changed, 112 insertions(+), 9 deletions(-)

LGTM.  Thanks for doing this!  :)

Reviewed-by: Doug Anderson <dianders@...omium.org>
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