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Date:   Fri, 3 Nov 2017 00:43:51 -0700
From:   Ramesh Thomas <ramesh.thomas@...el.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Tero Kristo <t-kristo@...com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Alex Shi <alex.shi@...aro.org>
Subject: Re: [RFT][PATCH 2/2] PM / QoS: Fix device resume latency framework

On 2017-11-02 at 00:03:54 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The special value of 0 for device resume latency PM QoS means
> "no restriction", but there are two problems with that.
> 
> First, device resume latency PM QoS requests with 0 as the
> value are always put in front of requests with positive
> values in the priority lists used internally by the PM QoS
> framework, causing 0 to be chosen as an effective constraint
> value.  However, that 0 is then interpreted as "no restriction"
> effectively overriding the other requests with specific
> restrictions which is incorrect.
> 
> Second, the users of device resume latency PM QoS have no
> way to specify that *any* resume latency at all should be
> avoided, which is an artificial limitation in general.
> 
> To address these issues, modify device resume latency PM QoS to
> use S32_MAX as the "no constraint" value and 0 as the "no
> latency at all" one and rework its users (the cpuidle menu
> governor, the genpd QoS governor and the runtime PM framework)
> to follow these changes.
> 
> Also add a special "n/a" value to the corresponding user space I/F
> to allow user space to indicate that it cannot accept any resume
> latencies at all for the given device.
> 
> Fixes: 85dc0b8a4019 (PM / QoS: Make it possible to expose PM QoS latency constraints)
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=197323
> Reported-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  Documentation/ABI/testing/sysfs-devices-power |    4 ++-
>  drivers/base/cpu.c                            |    3 +-
>  drivers/base/power/domain.c                   |    2 -
>  drivers/base/power/domain_governor.c          |   27 ++++++++++++--------------
>  drivers/base/power/qos.c                      |    5 +++-
>  drivers/base/power/runtime.c                  |    2 -
>  drivers/base/power/sysfs.c                    |   25 ++++++++++++++++++++----
>  drivers/cpuidle/governors/menu.c              |    4 +--
>  include/linux/pm_qos.h                        |   24 +++++++++++++++--------
>  9 files changed, 63 insertions(+), 33 deletions(-)
> 
> Index: linux-pm/drivers/base/power/sysfs.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/sysfs.c
> +++ linux-pm/drivers/base/power/sysfs.c
> @@ -218,7 +218,14 @@ static ssize_t pm_qos_resume_latency_sho
>  					  struct device_attribute *attr,
>  					  char *buf)
>  {
> -	return sprintf(buf, "%d\n", dev_pm_qos_requested_resume_latency(dev));
> +	s32 value = dev_pm_qos_requested_resume_latency(dev);
> +
> +	if (value == 0)
> +		return sprintf(buf, "n/a\n");
> +	else if (value == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
> +		value = 0;
> +
> +	return sprintf(buf, "%d\n", value);
>  }
>  
>  static ssize_t pm_qos_resume_latency_store(struct device *dev,
> @@ -228,11 +235,21 @@ static ssize_t pm_qos_resume_latency_sto
>  	s32 value;
>  	int ret;
>  
> -	if (kstrtos32(buf, 0, &value))
> -		return -EINVAL;
> +	if (!kstrtos32(buf, 0, &value)) {
> +		/*
> +		 * Prevent users from writing negative or "no constraint" values
> +		 * directly.
> +		 */
> +		if (value < 0 || value == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
> +			return -EINVAL;
>  
> -	if (value < 0)
> +		if (value == 0)
> +			value = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> +	} else if (!strcmp(buf, "n/a") || !strcmp(buf, "n/a\n")) {
> +		value = 0;
> +	} else {
>  		return -EINVAL;
> +	}
>  
>  	ret = dev_pm_qos_update_request(dev->power.qos->resume_latency_req,
>  					value);
> Index: linux-pm/include/linux/pm_qos.h
> ===================================================================
> --- linux-pm.orig/include/linux/pm_qos.h
> +++ linux-pm/include/linux/pm_qos.h
> @@ -27,16 +27,17 @@ enum pm_qos_flags_status {
>  	PM_QOS_FLAGS_ALL,
>  };
>  
> -#define PM_QOS_DEFAULT_VALUE -1
> +#define PM_QOS_DEFAULT_VALUE	(-1)
> +#define PM_QOS_LATENCY_ANY	S32_MAX
>  
>  #define PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE	(2000 * USEC_PER_SEC)
>  #define PM_QOS_NETWORK_LAT_DEFAULT_VALUE	(2000 * USEC_PER_SEC)
>  #define PM_QOS_NETWORK_THROUGHPUT_DEFAULT_VALUE	0
>  #define PM_QOS_MEMORY_BANDWIDTH_DEFAULT_VALUE	0
> -#define PM_QOS_RESUME_LATENCY_DEFAULT_VALUE	0
> +#define PM_QOS_RESUME_LATENCY_DEFAULT_VALUE	PM_QOS_LATENCY_ANY
> +#define PM_QOS_RESUME_LATENCY_NO_CONSTRAINT	PM_QOS_LATENCY_ANY
>  #define PM_QOS_LATENCY_TOLERANCE_DEFAULT_VALUE	0
>  #define PM_QOS_LATENCY_TOLERANCE_NO_CONSTRAINT	(-1)
> -#define PM_QOS_LATENCY_ANY			((s32)(~(__u32)0 >> 1))
>  
>  #define PM_QOS_FLAG_NO_POWER_OFF	(1 << 0)
>  
> @@ -173,7 +174,8 @@ static inline s32 dev_pm_qos_requested_f
>  static inline s32 dev_pm_qos_raw_read_value(struct device *dev)
>  {
>  	return IS_ERR_OR_NULL(dev->power.qos) ?
> -		0 : pm_qos_read_value(&dev->power.qos->resume_latency);
> +		PM_QOS_RESUME_LATENCY_NO_CONSTRAINT :
> +		pm_qos_read_value(&dev->power.qos->resume_latency);
>  }
>  #else
>  static inline enum pm_qos_flags_status __dev_pm_qos_flags(struct device *dev,
> @@ -183,9 +185,9 @@ static inline enum pm_qos_flags_status d
>  							s32 mask)
>  			{ return PM_QOS_FLAGS_UNDEFINED; }
>  static inline s32 __dev_pm_qos_read_value(struct device *dev)
> -			{ return 0; }
> +			{ return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT; }
>  static inline s32 dev_pm_qos_read_value(struct device *dev)
> -			{ return 0; }
> +			{ return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT; }
>  static inline int dev_pm_qos_add_request(struct device *dev,
>  					 struct dev_pm_qos_request *req,
>  					 enum dev_pm_qos_req_type type,
> @@ -231,9 +233,15 @@ static inline int dev_pm_qos_expose_late
>  			{ return 0; }
>  static inline void dev_pm_qos_hide_latency_tolerance(struct device *dev) {}
>  
> -static inline s32 dev_pm_qos_requested_resume_latency(struct device *dev) { return 0; }
> +static inline s32 dev_pm_qos_requested_resume_latency(struct device *dev)
> +{
> +	return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> +}
>  static inline s32 dev_pm_qos_requested_flags(struct device *dev) { return 0; }
> -static inline s32 dev_pm_qos_raw_read_value(struct device *dev) { return 0; }
> +static inline s32 dev_pm_qos_raw_read_value(struct device *dev)
> +{
> +	return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> +}
>  #endif
>  
>  #endif
> Index: linux-pm/drivers/cpuidle/governors/menu.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> +++ linux-pm/drivers/cpuidle/governors/menu.c
> @@ -298,8 +298,8 @@ static int menu_select(struct cpuidle_dr
>  		data->needs_update = 0;
>  	}
>  
> -	/* resume_latency is 0 means no restriction */
> -	if (resume_latency && resume_latency < latency_req)
> +	if (resume_latency < latency_req &&
> +	    resume_latency != PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
>  		latency_req = resume_latency;
>  
>  	/* Special case when user has set very strict latency requirement */
> Index: linux-pm/drivers/base/power/runtime.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/runtime.c
> +++ linux-pm/drivers/base/power/runtime.c
> @@ -253,7 +253,7 @@ static int rpm_check_suspend_allowed(str
>  	    || (dev->power.request_pending
>  			&& dev->power.request == RPM_REQ_RESUME))
>  		retval = -EAGAIN;
> -	else if (__dev_pm_qos_read_value(dev) < 0)
> +	else if (__dev_pm_qos_read_value(dev) == 0)
>  		retval = -EPERM;
>  	else if (dev->power.runtime_status == RPM_SUSPENDED)
>  		retval = 1;
> Index: linux-pm/drivers/base/cpu.c
> ===================================================================
> --- linux-pm.orig/drivers/base/cpu.c
> +++ linux-pm/drivers/base/cpu.c
> @@ -377,7 +377,8 @@ int register_cpu(struct cpu *cpu, int nu
>  
>  	per_cpu(cpu_sys_devices, num) = &cpu->dev;
>  	register_cpu_under_node(num, cpu_to_node(num));
> -	dev_pm_qos_expose_latency_limit(&cpu->dev, 0);
> +	dev_pm_qos_expose_latency_limit(&cpu->dev,
> +					PM_QOS_RESUME_LATENCY_NO_CONSTRAINT);
>  
>  	return 0;
>  }
> Index: linux-pm/drivers/base/power/qos.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/qos.c
> +++ linux-pm/drivers/base/power/qos.c
> @@ -139,6 +139,9 @@ static int apply_constraint(struct dev_p
>  
>  	switch(req->type) {
>  	case DEV_PM_QOS_RESUME_LATENCY:
> +		if (WARN_ON(value < 0))
> +			value = 0;
> +
>  		ret = pm_qos_update_target(&qos->resume_latency,
>  					   &req->data.pnode, action, value);
>  		break;
> @@ -189,7 +192,7 @@ static int dev_pm_qos_constraints_alloca
>  	plist_head_init(&c->list);
>  	c->target_value = PM_QOS_RESUME_LATENCY_DEFAULT_VALUE;
>  	c->default_value = PM_QOS_RESUME_LATENCY_DEFAULT_VALUE;
> -	c->no_constraint_value = PM_QOS_RESUME_LATENCY_DEFAULT_VALUE;
> +	c->no_constraint_value = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
>  	c->type = PM_QOS_MIN;
>  	c->notifiers = n;
>  
> Index: linux-pm/Documentation/ABI/testing/sysfs-devices-power
> ===================================================================
> --- linux-pm.orig/Documentation/ABI/testing/sysfs-devices-power
> +++ linux-pm/Documentation/ABI/testing/sysfs-devices-power
> @@ -211,7 +211,9 @@ Description:
>  		device, after it has been suspended at run time, from a resume
>  		request to the moment the device will be ready to process I/O,
>  		in microseconds.  If it is equal to 0, however, this means that
> -		the PM QoS resume latency may be arbitrary.
> +		the PM QoS resume latency may be arbitrary and the special value
> +		"n/a" means that user space cannot accept any resume latency at
> +		all for the given device.
>  
>  		Not all drivers support this attribute.  If it isn't supported,
>  		it is not present.
> Index: linux-pm/drivers/base/power/domain.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/domain.c
> +++ linux-pm/drivers/base/power/domain.c
> @@ -1331,7 +1331,7 @@ static struct generic_pm_domain_data *ge
>  
>  	gpd_data->base.dev = dev;
>  	gpd_data->td.constraint_changed = true;
> -	gpd_data->td.effective_constraint_ns = 0;
> +	gpd_data->td.effective_constraint_ns = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
>  	gpd_data->nb.notifier_call = genpd_dev_pm_qos_notifier;
>  
>  	spin_lock_irq(&dev->power.lock);
> Index: linux-pm/drivers/base/power/domain_governor.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/domain_governor.c
> +++ linux-pm/drivers/base/power/domain_governor.c
> @@ -26,11 +26,11 @@ static int dev_update_qos_constraint(str
>  	 * to take effect, the device has to be resumed and suspended again.
>  	 */
>  	constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
> -	/* 0 means "no constraint" */
> -	if (constraint_ns == 0)
> +	if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
>  		return 0;
>  
> -	if (constraint_ns < *constraint_ns_p || *constraint_ns_p == 0)
> +	if (constraint_ns < *constraint_ns_p ||
> +	    *constraint_ns_p == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)

Check for constraint_ns < *constraint_ns_p should be enough because
PM_QOS_RESUME_LATENCY_NO_CONSTRAINT = S32_MAX which is the largest possible
value, if that can be assumed here.

>  		*constraint_ns_p = constraint_ns;
>  
>  	return 0;
> @@ -58,12 +58,12 @@ static bool default_suspend_ok(struct de
>  	}
>  	td->constraint_changed = false;
>  	td->cached_suspend_ok = false;
> -	td->effective_constraint_ns = -1;
> +	td->effective_constraint_ns = 0;
>  	constraint_ns = __dev_pm_qos_read_value(dev);

Why not initialize td->effective_constraint_ns with what is read by
__dev_pm_qos_read_value()? Even though this value will stay only if false
is returned, in which case it does not matter anyway, it looks like more
meaningful value. Also PM_QOS_RESUME_LATENCY_NO_CONSTRAINT may be ok as it
is initialized with at creation.

>  
>  	spin_unlock_irqrestore(&dev->power.lock, flags);
>  
> -	if (constraint_ns < 0)
> +	if (constraint_ns == 0)
>  		return false;
>  
>  	constraint_ns *= NSEC_PER_USEC;

This will go wrong if PM_QOS_RESUME_LATENCY_NO_CONSTRAINT was the value
Code below and in dev_update_qos_constraint check for that special value.

May be those places can check PM_QOS_RESUME_LATENCY_NO_CONSTRAINT * NSEC_PER_USEC
There may be other places that this might be an issue

> @@ -76,14 +76,14 @@ static bool default_suspend_ok(struct de
>  		device_for_each_child(dev, &constraint_ns,
>  				      dev_update_qos_constraint);
>  
> -	if (constraint_ns == 0) {
> +	if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT) {
>  		/* "No restriction", so the device is allowed to suspend. */
> -		td->effective_constraint_ns = 0;
> +		td->effective_constraint_ns = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
>  		td->cached_suspend_ok = true;
> -	} else if (WARN_ON(constraint_ns < 0)) {
> +	} else if (WARN_ON(constraint_ns == 0)) {

This can never happen as this device will not be suspending if the child had
failed to suspend (only case where this value is possible).

>  		/*
> -		 * effective_constraint_ns is negative already and
> -		 * cached_suspend_ok is false, so return right away.
> +		 * effective_constraint_ns is 0 already and cached_suspend_ok is
> +		 * false, so return right away.
>  		 */
>  		return false;
>  	} else {
> @@ -156,12 +156,11 @@ static bool __default_power_down_ok(stru
>  		 */
>  		td = &to_gpd_data(pdd)->td;
>  		constraint_ns = td->effective_constraint_ns;
> -		/* Negative values mean "no suspend at all" */
> -		if (WARN_ON(constraint_ns < 0))
> +		/* 0 mean "no suspend at all" */
> +		if (WARN_ON(constraint_ns == 0))
>  			return false;

If it did not suspend then it cannot powerdown. This check may not be
very useful here.

>  
> -		/* 0 means "no constraint" */
> -		if (constraint_ns == 0)
> +		if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
>  			continue;
>  
>  		if (constraint_ns <= off_on_time_ns)
> 

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