Date:	Wed, 29 Jul 2015 18:04:56 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Viresh Kumar <viresh.kumar@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"mnipxh@....com" <mnipxh@....com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH] cpufreq: Add scaling frequency range support


On 2015年07月29日 17:59, Pan Xinhui wrote:
> hi, Rafael
> 	thanks for you reply.
> 
> On 2015年07月29日 08:18, Rafael J. Wysocki wrote:
>> On Tuesday, July 28, 2015 12:53:33 PM Pan Xinhui wrote:
>>> 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.
>>
>> Fix them, then, pretty please.
>>
> of course we will fix them. :)
> 
>> And adding this attribute is not going to magically fix them, is it?
>>
> yes, this patch can not fix them without changing the script. BUT I have another patch which could magically fix them. :)
> 
> These two attribute files are very tricky. they are related with each other.
> Not like some other attribute file in other part of kernel, for example, proc/sys/fs/file-max.
> As the file-min is always zero. It's very reasonable to only support file-max attribute file.
> 
> The sequence we echoing value to min/max_freq is very important. Maybe we can also assume they have *state*.
> Just like a developer writes a buf to a file. he should do in this way below.
> fp = fopen(..)
>  => fwrite(...)
>   => fclose(...)
> 
> The script I mentioned above did not follow the right sequence. when script wants to set the min higher, we need set the max first to avoid min > max issue...
> So max/min_freq have *state*. just like TCP Three-way handshake, SYN, ACK&SYN, ACK. the sequence(this is so-called state) is very important.
> 
> Now I want to offer a non-state attribute to user-space :)
> This is a design/engineering problem. It's okay for kernel to not offer such attribute. But user-space will do more work.
> For example, In the worst case, we need system call four times.
> read min/max_freq (system call two times)
> might set min or max freq first to avoid min > max issue (system call one time)
> set min/max a new value (system call one time)
> 
> What if we offer *set freq range* attribute? just once. :)
> set freq range (system call one time)
> 
> From performance point, It's a good idea to offer such attribute.
> 
> There is another reason for why it's good to apply this patch.
> If cpufreg range is 480000-960000, we call it powersave, 480000-2240000 is normal, 1920000-2240000 is performance.
> Assume current cpufreq range is powersave, then user want to set it to performance because user wants to play a 3D game.
> BUT user have to set it to normal first, then set it to performance because min(performance) > max(powersave).....
> I don't know how people(end-user) would think about such behavior.... why we must be back to normal first, then performance?
> 
> As for the patch I mentioned above which could magically fix them.
> The solution is: change store_scaling_max_freq and store_scaling_min_freq sysfs callback, let them have *state*.
> Always keep the value from user-space.
> 
> patch like:
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 8772346..00e6965 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -615,6 +615,14 @@ static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
>  static int cpufreq_set_policy(struct cpufreq_policy *policy,
>                                 struct cpufreq_policy *new_policy);
>  
> +static void
> +cpufreq_get_user_policy_freq(struct cpufreq_real_policy *user_policy,
> +                               struct cpufreq_policy *policy)
> +{
> +       policy->min = user_policy->min;
> +       policy->max = user_policy->max;
> +}
> +
>  /**
>   * cpufreq_per_cpu_attr_write() / store_##file_name() - sysfs write access
>   */
> @@ -622,21 +630,20 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>  static ssize_t store_##file_name                                       \
>  (struct cpufreq_policy *policy, const char *buf, size_t count)         \
>  {                                                                      \
> -       int ret, temp;                                                  \
> +       int ret;                                                        \
>         struct cpufreq_policy new_policy;                               \
>                                                                         \
>         ret = cpufreq_get_policy(&new_policy, policy->cpu);             \
>         if (ret)                                                        \
>                 return -EINVAL;                                         \
>                                                                         \
> +       cpufreq_get_user_policy_freq(&policy->user_policy, &new_policy);\
>         ret = sscanf(buf, "%u", &new_policy.object);                    \
>         if (ret != 1)                                                   \
>                 return -EINVAL;                                         \
>                                                                         \
> -       temp = new_policy.object;                                       \
> -       ret = cpufreq_set_policy(policy, &new_policy);          \
> -       if (!ret)                                                       \
> -               policy->user_policy.object = temp;                      \
> +       policy->user_policy.object = policy->object;                    \
should be 
+	policy->user_policy.object = new_policy.object;		\
sorry for that.
> +       ret = cpufreq_set_policy(policy, &new_policy);                  \
>                                                                         \
>         return ret ? ret : count;                                       \
>  }
> 
>

> 
> 
> Thanks
> xinhui
> 
>> Thanks,
>> Rafael
>>
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