linux-kernel - Re: [for-next PATCH] PM / devfreq: Add sysfs node to expose available frequencies

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Date:	Fri, 26 Oct 2012 01:56:44 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Nishanth Menon <nm@...com>
Cc:	linux-pm <linux-pm@...r.kernel.org>,
	Rajagopal Venkat <rajagopal.venkat@...aro.org>,
	MyungJoo Ham <myungjoo.ham@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Kevin Hilman <khilman@...com>, linux-kernel@...r.kernel.org
Subject: Re: [for-next PATCH] PM / devfreq: Add sysfs node to expose available frequencies

On Thursday, October 25, 2012 06:32:43 PM Nishanth Menon wrote:
> devfreq governors such as ondemand are controlled by a min and
> max frequency, while governors like userspace governor allow us
> to set a specific frequency.
> However, for the same specific device, depending on the SoC, the
> available frequencies can vary.
> 
> So expose the available frequencies as a snapshot over sysfs to
> allow informed decisions.
> 
> This was inspired by cpufreq framework's equivalent for similar
> usage sysfs node: scaling_available_frequencies.
> 
> Cc: Rajagopal Venkat <rajagopal.venkat@...aro.org>
> Cc: MyungJoo Ham <myungjoo.ham@...sung.com>
> Cc: Kyungmin Park <kyungmin.park@...sung.com>
> Cc: "Rafael J. Wysocki" <rjw@...k.pl>
> Cc: Kevin Hilman <khilman@...com>
> Cc: linux-pm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> 
> Signed-off-by: Nishanth Menon <nm@...com>
> ---
> Applies on top of Rafael's linux-next branch:
> git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git
>  linux-next                                   b960e9a Merge branch 'pm-sleep-next' into linux-next
> 
> Example output from Beagleboard XM (3730) using a dummy test driver
> http://pastebin.pandaboard.org/index.php/view/85100576 :
> 
> /sys/devices/platform/iva.0/devfreq/iva.0 # cat available_frequencies 
> 260000000 520000000 660000000 
> 
>  Documentation/ABI/testing/sysfs-class-devfreq |    9 +++++++++
>  drivers/devfreq/devfreq.c                     |   26 +++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-devfreq b/Documentation/ABI/testing/sysfs-class-devfreq
> index e6cf08e..e672ccb 100644
> --- a/Documentation/ABI/testing/sysfs-class-devfreq
> +++ b/Documentation/ABI/testing/sysfs-class-devfreq
> @@ -51,3 +51,12 @@ Description:
>  		The /sys/class/devfreq/.../userspace/set_freq shows and
>  		sets the requested frequency for the devfreq object if
>  		userspace governor is in effect.
> +
> +What:		/sys/class/devfreq/.../available_frequencies
> +Date:		October 2012
> +Contact:	Nishanth Menon <nm@...com>
> +Description:
> +		The /sys/class/devfreq/.../available_frequencies shows
> +		the available frequencies of the corresponding devfreq object.
> +		This is a snapshot of available frequencies and not limited
> +		by the min/max frequency restrictions.
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index d02ee7e..2f6ad6b 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -571,9 +571,35 @@ static ssize_t show_max_freq(struct device *dev, struct device_attribute *attr,
>  	return sprintf(buf, "%lu\n", to_devfreq(dev)->max_freq);
>  }
>  
> +static ssize_t show_available_freqs(struct device *d,
> +				    struct device_attribute *attr,
> +				    char *buf)
> +{
> +	struct devfreq *df = to_devfreq(d);
> +	struct device *dev = df->dev.parent;
> +	struct opp *opp;
> +	ssize_t count = 0;
> +	unsigned long freq = 0;
> +
> +	rcu_read_lock();
> +	do {
> +		opp = opp_find_freq_ceil(dev, &freq);
> +		if (IS_ERR(opp))
> +			break;
> +
> +		count += sprintf(&buf[count], "%lu ", freq);
> +		freq++;
> +	} while (1);
> +	rcu_read_unlock();
> +	count += sprintf(&buf[count], "\n");

Care to avoid printing the tailing space?

Rafael


> +
> +	return count;
> +}
> +
>  static struct device_attribute devfreq_attrs[] = {
>  	__ATTR(governor, S_IRUGO, show_governor, NULL),
>  	__ATTR(cur_freq, S_IRUGO, show_freq, NULL),
> +	__ATTR(available_frequencies, S_IRUGO, show_available_freqs, NULL),
>  	__ATTR(target_freq, S_IRUGO, show_target_freq, NULL),
>  	__ATTR(polling_interval, S_IRUGO | S_IWUSR, show_polling_interval,
>  	       store_polling_interval),
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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