linux-kernel - Re: [PATCH v2 12/19] PM / devfreq: tegra: Avoid inconsistency of current frequency value

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Date:   Tue, 16 Apr 2019 16:15:10 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 12/19] PM / devfreq: tegra: Avoid inconsistency of
 current frequency value

Hi,

On 19. 4. 15. 오후 11:54, Dmitry Osipenko wrote:
> The frequency value potentially could change in-between. It doesn't
> cause any real problem at all right now, but that could change in the
> future. Hence let's avoid the inconsistency.
> 
> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> ---
>  drivers/devfreq/tegra-devfreq.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/devfreq/tegra-devfreq.c b/drivers/devfreq/tegra-devfreq.c
> index a668e4fbc874..f1a6f951813a 100644
> --- a/drivers/devfreq/tegra-devfreq.c
> +++ b/drivers/devfreq/tegra-devfreq.c
> @@ -496,13 +496,15 @@ static int tegra_devfreq_get_dev_status(struct device *dev,
>  {
>  	struct tegra_devfreq *tegra = dev_get_drvdata(dev);
>  	struct tegra_devfreq_device *actmon_dev;
> +	unsigned long cur_freq;
>  
> -	stat->current_frequency = tegra->cur_freq * KHZ;
> +	cur_freq = READ_ONCE(tegra->cur_freq);
>  
>  	/* To be used by the tegra governor */
>  	stat->private_data = tegra;
>  
>  	/* The below are to be used by the other governors */
> +	stat->current_frequency = cur_freq * KHZ;
>  
>  	actmon_dev = &tegra->devices[MCALL];
>  
> @@ -513,7 +515,7 @@ static int tegra_devfreq_get_dev_status(struct device *dev,
>  	stat->busy_time *= 100 / BUS_SATURATION_RATIO;
>  
>  	/* Number of cycles in a sampling period */
> -	stat->total_time = ACTMON_SAMPLING_PERIOD * tegra->cur_freq;
> +	stat->total_time = ACTMON_SAMPLING_PERIOD * cur_freq;
>  
>  	stat->busy_time = min(stat->busy_time, stat->total_time);
>  
> 

The read/write access of tegra->cur_freq is in the single routine
of update_devfreq() as following. I think that there are no any
potential problem about the inconsistency of tegra->cur_freq.

IMHO, if there are no any problem now, I'm not sure that we need
to apply this patch.

update_devfreq()
{
	devfreq->governor->get_target_freq()
		devfreq_update_stats(devfreq)
			tegra_devfreq_get_dev_status()
				stat->current_frequency = tegra->cur_freq * KHZ;

	devfreq_set_target()
		tegra_devfreq_target()
			clk_set_min_rate(emc_rate, )
				tegra_actmon_rate_notify_cb()
					tegra->cur_freq = data->new_rate / KHZ;
		
			clk_set_rate(emc_rate, )
				tegra_actmon_rate_notify_cb()
					tegra->cur_freq = data->new_rate / KHZ;
}


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics