Re: [PATCH v4 1/3] PM: Introduce DEVFREQ: generic DVFS framework with device-specific OPPs

[Kevin Hilman <khilman@ti.com>]

MyungJoo Ham <myungjoo.ham@samsung.com> writes:

> With OPPs, a device may have multiple operable frequency and voltage
> sets. However, there can be multiple possible operable sets and a system
> will need to choose one from them. In order to reduce the power
> consumption (by reducing frequency and voltage) without affecting the
> performance too much, a Dynamic Voltage and Frequency Scaling (DVFS)
> scheme may be used.
>
> This patch introduces the DVFS capability to non-CPU devices with OPPs.
> DVFS is a techique whereby the frequency and supplied voltage of a
> device is adjusted on-the-fly. DVFS usually sets the frequency as low
> as possible with given conditions (such as QoS assurance) and adjusts
> voltage according to the chosen frequency in order to reduce power
> consumption and heat dissipation.
>
> The generic DVFS for devices, DEVFREQ, may appear quite similar with
> /drivers/cpufreq.  However, CPUFREQ does not allow to have multiple
> devices registered and is not suitable to have multiple heterogenous
> devices with different (but simple) governors.
>
> Normally, DVFS mechanism controls frequency based on the demand for
> the device, and then, chooses voltage based on the chosen frequency.
> DEVFREQ also controls the frequency based on the governor's frequency
> recommendation and let OPP pick up the pair of frequency and voltage
> based on the recommended frequency. Then, the chosen OPP is passed to
> device driver's "target" callback.
>
> Tested with memory bus of Exynos4-NURI board.
>
> The test code with board support for Exynos4-NURI is at
> http://git.infradead.org/users/kmpark/linux-2.6-samsung/shortlog/refs/heads/devfreq
>
> Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>

[...]

> +int devfreq_update(struct device *dev)
> +{
> +	struct devfreq *devfreq;
> +	int err = 0;
> +
> +	mutex_lock(&devfreq_list_lock);
> +
> +	devfreq = find_device_devfreq(dev);
> +	if (IS_ERR(devfreq)) {
> +		err = PTR_ERR(devfreq);
> +		goto out;
> +	}
> +
> +	/*
> +	 * If the maximum frequency available is changed either by
> +	 * enabling higher frequency or disabling the current
> +	 * maximum frequency, we need to adjust the frequency
> +	 * (tickle) again if the device has been being tickled.
> +	 */
> +	if (devfreq->tickle) {
> +		unsigned long freq = devfreq->profile->max_freq;
> +		struct opp *opp = opp_find_freq_floor(devfreq->dev, &freq);
> +
> +		if (IS_ERR(opp)) {
> +			err = PTR_ERR(opp);
> +			goto out;
> +		}
> +
> +		/* Max freq available is not changed */
> +		if (devfreq->previous_freq == freq)
> +			goto out;
> +
> +		/* Tickle again. Max freq available is changed */
> +		err = devfreq->profile->target(devfreq->dev, opp);
> +		if (!err)
> +			devfreq->previous_freq = freq;

This looks an awful lot like _devfreq_tickle_device()... wondering why
that is not called here.

> +	} else {
> +		/* Reevaluate the proper frequency */
> +		err = devfreq_do(devfreq);
> +	}
> +
> +out:
> +	mutex_unlock(&devfreq_list_lock);
> +	return err;
> +}

Kevin