linux-kernel - Re: [PATCH] cpufreq: Add scaling frequency range support

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Date:	Tue, 28 Jul 2015 12:53:33 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"mnipxh@....com" <mnipxh@....com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH] cpufreq: Add scaling frequency range support

hi, Viresh
	thanks for your reply :)
On 2015年07月28日 12:29, Viresh Kumar wrote:
> On 28-07-15, 11:32, Pan Xinhui wrote:
>> From: Pan Xinhui <xinhuix.pan@...el.com>
>>
>> Userspace at most time do cpufreq tests very much inconveniently.
>> Currently they have to echo min and max cpu freq separately like below:
>> echo 480000  > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
>> echo 2240000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
>>
>> Add scaling_freq_range cpufreq attr to support userspace's demand.
>> Therefore it's easier for testers to write readable scripts like below: 
>> echo 480000-2240000 >
>> /sys/devices/system/cpu/cpu0/cpufreq/scaling_freq_range
> 
> I don't think this brings any good change, we already have support for
> that with min/max freqs and I don't see how scripts can be less
> readable with that.
> 
yes, min/max are supported, however it is inconvenient. sometime it's very easy to cause obscure bugs.
For example, some one might write a script like below.
echo 480000  > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
echo 960000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
.....//other works
echo 1120000  > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
echo 2240000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
...//other works

But it did not work when we echo 112000 to min-freq, as the current max freq is smaller than it.
It's hard to figure it out in a big script... we have many such scripts.
I admit this is a bug in script. If we can support *set freq range*, it's a very good option for usesapce to change the cpufreq.
People working in useespace will be thankful to us :)

> So, why to add redundant files at all? Also note that we can't remove
> the old interface as that will break the ABI.
> 
fully agree! we can't break the ABI. So i just add this feature which is very helpful. :)


thanks
xinhui
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