linux-kernel - Re: [PATCH v4 14/24] PM / devfreq: tegra30: Ensure that target freq won't overflow

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Message-ID: <14115aa7-ed40-2775-9b0b-a2bf6ca9d47e@gmail.com>
Date:   Tue, 16 Jul 2019 16:59:42 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Chanwoo Choi <cw00.choi@...sung.com>,
        Thierry Reding <thierry.reding@...il.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 14/24] PM / devfreq: tegra30: Ensure that target freq
 won't overflow

16.07.2019 15:30, Chanwoo Choi пишет:
> On 19. 7. 8. 오전 7:32, Dmitry Osipenko wrote:
>> Potentially very high boosting could cause an integer overflow for a
>> highly clocked memory after conversion to MHz.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>> ---
>>  drivers/devfreq/tegra30-devfreq.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/devfreq/tegra30-devfreq.c b/drivers/devfreq/tegra30-devfreq.c
>> index 2f59c78930bd..0de1efdaabf4 100644
>> --- a/drivers/devfreq/tegra30-devfreq.c
>> +++ b/drivers/devfreq/tegra30-devfreq.c
>> @@ -460,6 +460,7 @@ static unsigned long actmon_update_target(struct tegra_devfreq *tegra,
>>  	unsigned long target_freq;
>>  
>>  	target_freq = dev->avg_count / ACTMON_SAMPLING_PERIOD + dev->boost_freq;
>> +	target_freq = min(target_freq, ULONG_MAX / KHZ);
> 
> Did you meet this corner case?

I can't recall that, technically it could happen and I don't feel
comfortable by having potential integer overflows anyway.

> If have to change it, you better to use 'tegra->max_freq' as following:
> 	min(target_freq, tegra->max_freq);

The 'tegra->max_freq' will work here, but 'ULONG_MAX / KHZ' is a
constant value which is also correct and doesn't case any harm.

Probably will be even more expressive to write this as:

	target_freq = dev->avg_freq + dev->boost_freq;
	target_freq = tegra_actmon_account_cpu_freq(tegra, dev, target_freq);

	return min(target_freq, tegra->max_freq);


Thank you very much for the reviews!