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Date:   Fri, 3 Nov 2017 08:58:47 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     ramesh.thomas@...el.com
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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 Fri, Nov 3, 2017 at 8:43 AM, Ramesh Thomas <ramesh.thomas@...el.com> wrote:
> 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.

Well, not quite.  effective_constraint_ns is s64 and it is the QoS
value multiplied by NSEC_PER_USEC.

Surely can be greater than PM_QOS_RESUME_LATENCY_NO_CONSTRAINT.

Which reminds me about the possible collision in default_suspend_ok()
which needs to be addressed. :-)

>>               *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.

This looks like a leftover or similar, thanks.

>>
>>       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

Right, I forgot about this one.

>> @@ -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.

The checks with WARN_ON() are there to catch the unexpected.  They
aren't meant to trigger at all under normal conditions.

Thanks,
Rafael

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