linux-kernel - Re: [PATCH V4 3/3] cpufreq: Tegra: implement intermediate frequency callbacks

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Message-ID: <53877199.5090009@wwwdotorg.org>
Date:	Thu, 29 May 2014 11:42:49 -0600
From:	Stephen Warren <swarren@...dotorg.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 Mailing List <linux-kernel@...r.kernel.org>,
	Arvind Chauhan <arvind.chauhan@....com>,
	Stephen Warren <swarren@...dia.com>,
	Doug Anderson <dianders@...omium.org>,
	Russell King - ARM Linux <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 V4 3/3] cpufreq: Tegra: implement intermediate frequency
 callbacks

On 05/22/2014 10:05 PM, Viresh Kumar wrote:
> On 22 May 2014 22:09, Stephen Warren <swarren@...dotorg.org> wrote:
>> I think the call to tegra_target_intermediate() is wrong here; shouldn't
>> the cpufreq core guarantee that tegra_target_intermediate() has always
>> been called before tegra_target(), so there's no need to repeat that
>> call here?

>> Also, tegra_target() doesn't seem to follow the rule documented by patch
>> 2/3 that states ->target() should restore the orignal frequency on
>> error. I'm not even sure if that's possible in general.
> 
> I thought I took care of that. Can you please give some example when
> we aren't restoring original frequency on failure ?
> 
> About the rule, that has to be the expectation from core as there is no
> way out that for core to know what happened at end of target_index()..
> 
> It can call get_rate() but that would be over engineering it looks ..

E.g. in the following code after the series is applied:

> 	ret = clk_set_rate(pll_x_clk, rate * 1000);
> 	/* Restore to earlier frequency on error, i.e. pll_x */
> 	if (ret)
> 		pr_err("Failed to change pll_x to %lu\n", rate);
> 
> 	ret = clk_set_parent(cpu_clk, pll_x_clk);

If the clk_set_rate() failed, we have no idea what the pll_x rate is; I
don't think the clock API guarantees that zero HW changes have been made
if the function fails. Yet the code switches the CPU clock back to pll_x
without attempting to fix the pll_x rate to be the old rate. Perhaps
there's not much that can be done here though, since if one
clk_set_rate() failed, perhaps the one to fix it will too.

I suspect there are other issues, like switching between pll_p/pllx
might not be restored on error, and the EMC frequency isn't switched
back, etc.
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