Re: [PATCH v4] PM / devfreq: add PM-QoS support

[Rafael J. Wysocki]

On Monday 08 of October 2012 19:59:13 MyungJoo Ham wrote:
> Even if the performance of a device is controlled properly with devfreq,
> sometimes, we still need to get PM-QoS inputs in order to meet the
> required performance.
> 
> In our testbed of Exynos4412, which has on-chip various DMA devices, the
> memory interface and system bus are controlled according to their
> utilization by devfreq. However, in some multimedia applications
> including video-playing with MFC (multi-function codec) H/W and
> photo/video-capturing with FIMC H/W, we have observed issues due to
> insufficient DMA throughput or latency.
> 
> In such applications, there are deadlines: less than 16.6ms with 60Hz.
> With shorter polling intervals (5 ~ 15ms), the frequencies fluctuate
> within a frame and we get missing frames and distorted pictures.
> With longer polling intervals (20 ~ 100ms), the response time is not
> sufficient and we get distorted or broken images. In other words,
> regardless of polling interval, we get poor results with hard-deadline
> H/Ws. They, in fact, have a preset requirement on DMA throughput.
> 
> Thus, we need PM-QoS capabilities in devfreq. (Note that for general
> user applications, devfreq for bus/memory works fine. They are not so
> sensitive to tens of ms in performance increasing responses in general.
> 
> In order to express how to handle QoS requests in devfreq devices,
> the devfreq device drivers only need to express the mappings of
> QoS value and frequency pairs with QoS class along with
> devfreq_add_device() call.
> 
> Tested on Exynos4412 machines with memory/bus frequencies and multimedia
> H/W blocks. (camera, video decoding, and video encoding)
> 
> Signed-off-by: MyungJoo Ham <myungjoo.ham@xxxxxxxxxxx>
> Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>

So, there's one big problem I have with this approach.

If user space writes a number into the given PM QoS class' misc dev,
the written value is passed to the driver (through the notifier) and
the driver maps a frequency to it.  This means that in order to obtain
the desired performance, user space will need to know the mapping between
the QoS values and frequencies in the driver.  Now, it very well may happen
that two different drivers for two analogous devices will define PM QoS
tables mapping the QoS values to frequencies in different ways, so user
space will potentially need to know what driver is in use to get what it
wants.  That doesn't make a good interface.

Thanks,
Rafael


> ---
> Changes from V3
> - Corrected and added comments
> - Code Clean
> - Merged per-dev qos patch and global qos patch
> 
> Changed from V2-resend
> - Removed dependencies on global pm-qos class definitions
> - Revised data structure handling pm-qos (being ready for dev-pm-qos)
> 
> Changes from V2
> - Rebased
> 
> Changes from V1
> - Error handling at devfreq_add_device()
> - Handling pm_qos_max information
> - Styly update
> ---
>  drivers/devfreq/devfreq.c |  137 ++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/devfreq.h   |   41 +++++++++++++
>  2 files changed, 177 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 00e326c..d0cb33b 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -25,6 +25,7 @@
>  #include <linux/list.h>
>  #include <linux/printk.h>
>  #include <linux/hrtimer.h>
> +#include <linux/pm_qos.h>
>  #include "governor.h"
>  
>  static struct class *devfreq_class;
> @@ -136,8 +137,13 @@ int update_devfreq(struct devfreq *devfreq)
>  	 * List from the highest proiority
>  	 * max_freq (probably called by thermal when it's too hot)
>  	 * min_freq
> +	 * qos_min_freq
>  	 */
>  
> +	if (devfreq->qos_min_freq && freq < devfreq->qos_min_freq) {
> +		freq = devfreq->qos_min_freq;
> +		flags &= ~DEVFREQ_FLAG_LEAST_UPPER_BOUND; /* Use GLB */
> +	}
>  	if (devfreq->min_freq && freq < devfreq->min_freq) {
>  		freq = devfreq->min_freq;
>  		flags &= ~DEVFREQ_FLAG_LEAST_UPPER_BOUND; /* Use GLB */
> @@ -183,6 +189,56 @@ static int devfreq_notifier_call(struct notifier_block *nb, unsigned long type,
>  }
>  
>  /**
> + * devfreq_qos_notifier_call() - Handle QoS requests (updates)
> + *
> + * @nb		the notifier_block (supposed to be devfreq->qos_nb)
> + * @value	the QoS value
> + * @devp	not used
> + */
> +static int devfreq_qos_notifier_call(struct notifier_block *nb,
> +				     unsigned long value, void *devp)
> +{
> +	struct devfreq *devfreq = container_of(nb, struct devfreq, qos_nb);
> +	int ret;
> +	int i;
> +	s32 default_value = PM_QOS_DEFAULT_VALUE;
> +	struct devfreq_pm_qos_table *qos_list = devfreq->profile->qos_list;
> +	bool qos_use_max = devfreq->profile->qos_use_max;
> +
> +	if (!qos_list)
> +		return NOTIFY_DONE;
> +
> +	mutex_lock(&devfreq->lock);
> +
> +	if (value == default_value) {
> +		devfreq->qos_min_freq = 0;
> +		goto update;
> +	}
> +
> +	for (i = 0; qos_list[i].freq; i++) {
> +		/* QoS Met */
> +		if (qos_use_max) {
> +			if (qos_list[i].qos_value < value)
> +				continue;
> +		} else {
> +			if (qos_list[i].qos_value > value)
> +				continue;
> +		}
> +		devfreq->qos_min_freq = qos_list[i].freq;
> +		goto update;
> +	}
> +
> +	/* Use the highest QoS freq */
> +	devfreq->qos_min_freq = qos_list[i - 1].freq;
> +
> +update:
> +	ret = update_devfreq(devfreq);
> +	mutex_unlock(&devfreq->lock);
> +
> +	return ret;
> +}
> +
> +/**
>   * _remove_devfreq() - Remove devfreq from the device.
>   * @devfreq:	the devfreq struct
>   * @skip:	skip calling device_unregister().
> @@ -219,6 +275,13 @@ static void _remove_devfreq(struct devfreq *devfreq, bool skip)
>  
>  	devfreq->being_removed = true;
>  
> +	if (devfreq->profile->enable_dev_pm_qos)
> +		dev_pm_qos_remove_notifier(devfreq->dev.parent,
> +					   &devfreq->qos_nb);
> +	if (devfreq->profile->qos_type)
> +		pm_qos_remove_notifier(devfreq->profile->qos_type,
> +				       &devfreq->qos_nb);
> +
>  	if (devfreq->profile->exit)
>  		devfreq->profile->exit(devfreq->dev.parent);
>  
> @@ -376,6 +439,50 @@ static void devfreq_monitor(struct work_struct *work)
>  	}
>  }
>  
> +static int devfreq_qos_sanity_check(struct device *dev,
> +				    struct devfreq_dev_profile *profile)
> +{
> +	int i;
> +
> +	if (!profile->qos_type && !profile->enable_dev_pm_qos ||
> +				  !profile->qos_list) {
> +		dev_WARN(dev, "QoS requirement partially omitted.");
> +		return -EINVAL;
> +	}
> +
> +	if (!profile->qos_list[0].freq) {
> +		dev_WARN(dev, "The first QoS requirement is also the last requirement.");
> +		return -EINVAL;
> +	}
> +
> +	for (i = 1; profile->qos_list[i].freq; i++) {
> +		if (profile->qos_list[i].freq <=
> +		    profile->qos_list[i - 1].freq) {
> +			dev_WARN(dev, "qos_list[].freq not sorted in the ascending order. ([%d]=%lu, [%d]=%lu)\n",
> +				 i - 1, profile->qos_list[i - 1].freq,
> +				 i, profile->qos_list[i].freq);
> +			return -EINVAL;
> +
> +		if (profile->qos_use_max) {
> +			/* Throughput-like QoS type */
> +			if (profile->qos_list[i - 1].qos_value >
> +			    profile->qos_list[i].qos_value) {
> +				dev_WARN(dev, "qos_list[].qos_value needs to be sorted in the ascending order if qos_use_max is true.");
> +				return -EINVAL;
> +			}
> +		} else {
> +			/* Latency-like QoS type */
> +			if (profile->qos_list[i - 1].qos_value <
> +			    profile->qos_list[i].qos_value) {
> +				dev_WARN(dev, "qos_list[].qos_value needs to be sorted in the descending order if qos_use_max is false.");
> +				return -EINVAL;
> +			}
> +		}
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * devfreq_add_device() - Add devfreq feature to the device
>   * @dev:	the device to add devfreq feature.
> @@ -429,6 +536,21 @@ struct devfreq *devfreq_add_device(struct device *dev,
>  	devfreq->next_polling = devfreq->polling_jiffies
>  			      = msecs_to_jiffies(devfreq->profile->polling_ms);
>  	devfreq->nb.notifier_call = devfreq_notifier_call;
> +	devfreq->qos_nb.notifier_call = devfreq_qos_notifier_call;
> +
> +	/* Check the sanity of qos_list/qos_type */
> +	if (profile->qos_type || profile->enable_dev_pm_qos ||
> +	    profile->qos_list) {
> +		err = devfreq_qos_sanity_check(dev, profile);
> +		if (err)
> +			goto err_dev;
> +
> +		if (profile->qos_type)
> +			pm_qos_add_notifier(profile->qos_type,
> +					    &devfreq->qos_nb);
> +		if (profile->enable_dev_pm_qos)
> +			dev_pm_qos_add_notifier(dev, &devfreq->qos_nb);
> +	}
>  
>  	devfreq->trans_table =	devm_kzalloc(dev, sizeof(unsigned int) *
>  						devfreq->profile->max_state *
> @@ -443,7 +565,7 @@ struct devfreq *devfreq_add_device(struct device *dev,
>  	err = device_register(&devfreq->dev);
>  	if (err) {
>  		put_device(&devfreq->dev);
> -		goto err_dev;
> +		goto err_qos_add;
>  	}
>  
>  	if (governor->init)
> @@ -471,6 +593,11 @@ out:
>  
>  err_init:
>  	device_unregister(&devfreq->dev);
> +err_qos_add:
> +	if (profile->enable_dev_pm_qos)
> +		dev_pm_qos_remove_notifier(dev, &devfreq->qos_nb);
> +	if (profile->qos_type)
> +		pm_qos_remove_notifier(profile->qos_type, &devfreq->qos_nb);
>  err_dev:
>  	mutex_unlock(&devfreq->lock);
>  	kfree(devfreq);
> @@ -568,6 +695,13 @@ static ssize_t show_central_polling(struct device *dev,
>  		       !to_devfreq(dev)->governor->no_central_polling);
>  }
>  
> +static ssize_t show_qos_min_freq(struct device *dev,
> +				 struct device_attribute *attr,
> +				 char *buf)
> +{
> +	return sprintf(buf, "%lu\n", to_devfreq(dev)->qos_min_freq);
> +}
> +
>  static ssize_t store_min_freq(struct device *dev, struct device_attribute *attr,
>  			      const char *buf, size_t count)
>  {
> @@ -685,6 +819,7 @@ static struct device_attribute devfreq_attrs[] = {
>  	       store_polling_interval),
>  	__ATTR(min_freq, S_IRUGO | S_IWUSR, show_min_freq, store_min_freq),
>  	__ATTR(max_freq, S_IRUGO | S_IWUSR, show_max_freq, store_max_freq),
> +	__ATTR(qos_min_freq, S_IRUGO, show_qos_min_freq, NULL),
>  	__ATTR(trans_stat, S_IRUGO, show_trans_table, NULL),
>  	{ },
>  };
> diff --git a/include/linux/devfreq.h b/include/linux/devfreq.h
> index 30dc0d8..2488343 100644
> --- a/include/linux/devfreq.h
> +++ b/include/linux/devfreq.h
> @@ -53,6 +53,21 @@ struct devfreq_dev_status {
>  #define DEVFREQ_FLAG_LEAST_UPPER_BOUND		0x1
>  
>  /**
> + * struct devfreq_pm_qos_table - An PM QoS requiement entry for devfreq dev.
> + * @freq		Lowest frequency to meet the QoS requirement
> + *			represented by qos_value. If freq=0, it means that
> + *			this element is the last in the array.
> + * @qos_value		The qos value defined in pm_qos.h
> + *
> + * Note that the array of devfreq_pm_qos_table should be sorted by freq
> + * in the ascending order except for the last element, which should be 0.
> + */
> +struct devfreq_pm_qos_table {
> +	unsigned long freq; /* 0 if this is the last element */
> +	s32 qos_value;
> +};
> +
> +/**
>   * struct devfreq_dev_profile - Devfreq's user device profile
>   * @initial_freq	The operating frequency when devfreq_add_device() is
>   *			called.
> @@ -71,11 +86,33 @@ struct devfreq_dev_status {
>   *			from devfreq_remove_device() call. If the user
>   *			has registered devfreq->nb at a notifier-head,
>   *			this is the time to unregister it.
> + * @qos_type		QoS Type (defined in pm_qos.h)
> + *			0 (PM_QOS_RESERVED) if not used.
> + * @qos_use_max		True: the highest QoS value is used (for QoS
> + *			requirement of throughput, bandwidth, or similar)
> + *			False: the lowest QoS value is used (for QoS
> + *			requirement of latency, delay, or similar)
> + * @enable_dev_pm_qos	dev_pm_qos is enabled using the qos_list.
> + * @qos_list		Array of QoS requirements ending with .freq = 0
> + *			NULL if not used. It should be either NULL or
> + *			have a length > 1 with a first element effective.
> + *			This QoS specification is shared by the global
> + *			PM QoS (qos_type) and the per-dev PM QoS
> + *			(enable_dev_pm_qos).
> + *
> + * Note that the array of qos_list should be sorted by freq
> + * in the ascending order.
>   */
>  struct devfreq_dev_profile {
>  	unsigned long initial_freq;
>  	unsigned int polling_ms;
>  
> +	/* Optional QoS Handling Specification */
> +	int qos_type; /* 0: No global PM-QoS support */
> +	bool qos_use_max; /* true if "bandwidth"/"throughput"-like values */
> +	bool enable_dev_pm_qos; /* False: No per-dev PM-QoS support */
> +	struct devfreq_pm_qos_table *qos_list; /* QoS handling specification */
> +
>  	int (*target)(struct device *dev, unsigned long *freq, u32 flags);
>  	int (*get_dev_status)(struct device *dev,
>  			      struct devfreq_dev_status *stat);
> @@ -139,6 +176,8 @@ struct devfreq_governor {
>   *			order to prevent trying to remove the object multiple times.
>   * @min_freq	Limit minimum frequency requested by user (0: none)
>   * @max_freq	Limit maximum frequency requested by user (0: none)
> + * @qos_nb	notifier block used to notify pm qos requests
> + * @qos_min_freq	Limit minimum frequency requested by QoS
>   *
>   * This structure stores the devfreq information for a give device.
>   *
> @@ -167,6 +206,8 @@ struct devfreq {
>  
>  	unsigned long min_freq;
>  	unsigned long max_freq;
> +	struct notifier_block qos_nb;
> +	unsigned long qos_min_freq;
>  
>  	/* information for device freqeuncy transition */
>  	unsigned int total_trans;
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/