Re: [PATCH v6 08/19] PM / devfreq: tegra30: Ensure that target freq won't overflow

[Dmitry Osipenko]

Hello Chanwoo,

20.08.2019 3:23, Chanwoo Choi ÐÐÑÐÑ:
> On 19. 8. 12. ìì 6:23, Dmitry Osipenko wrote:
>> We already had few integer overflow bugs, let's limit the freq for
>> consistency.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
>> ---
>>  drivers/devfreq/tegra30-devfreq.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/devfreq/tegra30-devfreq.c b/drivers/devfreq/tegra30-devfreq.c
>> index 70dce58212a4..ca499368ee81 100644
>> --- a/drivers/devfreq/tegra30-devfreq.c
>> +++ b/drivers/devfreq/tegra30-devfreq.c
>> @@ -430,7 +430,7 @@ static unsigned long actmon_update_target(struct tegra_devfreq *tegra,
>>  	target_freq = dev->avg_count / ACTMON_SAMPLING_PERIOD + dev->boost_freq;
>>  	target_freq = tegra_actmon_account_cpu_freq(tegra, dev, target_freq);
>>  
>> -	return target_freq;
>> +	return min(target_freq, tegra->max_freq);
> 
> Once again, did you meet this case sometimes?

This case happens at least whenever CPU's freq accounting returns
KHZ_MAX, please see actmon_emc_ratios[] and actmon_cpu_to_emc_rate().

> Usually, we can prevent the overflow of target_freq
> when calculating the target frequency or this style.
> 
> I think that if the overflow of target frequency happen frequently,
> it might have the problem of calculation way. 

It is possible to get sampled avg_count from hardware that results in
target_freq > max_freq because translating number of memory events to
memory frequency has some calculation error, although it is negligible.

In fact, it shouldn't matter that the returned target_freq > max_freq
with the current driver's code structure and in the commit's message I
wrote that this change is done for consistency. Hence it should be okay
to drop this patch as well.