[PATCH 1/2] PM-runtime: Documentation: add runtime_status ABI document

[PATCH 1/2] PM-runtime: Documentation: add runtime_status ABI document

[Akinobu Mita]

/sys/devices/.../power/runtime_status is introduced by commit c92445fadb91
("PM / Runtime: Add sysfs debug files").  Then commit 0fcb4eef8294 ("PM /
Runtime: Make runtime_status attribute not debug-only (v. 2)") made the
runtime_status attribute available without CONFIG_PM_ADVANCED_DEBUG.

This adds missing runtime_status ABI document.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 Documentation/ABI/testing/sysfs-devices-power | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
index 80a00f7..3e50536 100644
--- a/Documentation/ABI/testing/sysfs-devices-power
+++ b/Documentation/ABI/testing/sysfs-devices-power
@@ -260,3 +260,12 @@ Description:
 
 		This attribute has no effect on system-wide suspend/resume and
 		hibernation.
+
+What:		/sys/devices/.../power/runtime_status
+Date:		April 2010
+Contact:	Dominik Brodowski <linux@dominikbrodowski.net>
+Description:
+		The /sys/devices/.../power/runtime_status attribute contains
+		the current runtime PM status of the device, which may be
+		"suspended", "suspending", "resuming", "active", "error" (fatal
+		error), or "unsupported" (runtime PM is disabled).
-- 
2.7.4

[PATCH 2/2] PM-runtime: allow userspace to monitor runtime_status changes

[Akinobu Mita]

This enables the /sys/devices/.../power/runtime_status attribute to
allow the user space to get notifications via poll/select when the device
runtime PM status is changed.

An example use case is to avoid unnecessary accesses for device statistics
(e.g. diskstats for block devices) while the device is in runtime suspend
by user space LED device actitity trigger.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 Documentation/ABI/testing/sysfs-devices-power | 2 ++
 drivers/base/power/power.h                    | 1 +
 drivers/base/power/runtime.c                  | 1 +
 drivers/base/power/sysfs.c                    | 5 +++++
 4 files changed, 9 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
index 3e50536..47dc357 100644
--- a/Documentation/ABI/testing/sysfs-devices-power
+++ b/Documentation/ABI/testing/sysfs-devices-power
@@ -269,3 +269,5 @@ Description:
 		the current runtime PM status of the device, which may be
 		"suspended", "suspending", "resuming", "active", "error" (fatal
 		error), or "unsupported" (runtime PM is disabled).
+		This attribute allows the user space to get notifications via
+		poll/select when the device runtime PM status is changed.
diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
index ec33fbdb..8891bf4 100644
--- a/drivers/base/power/power.h
+++ b/drivers/base/power/power.h
@@ -74,6 +74,7 @@ extern int pm_qos_sysfs_add_flags(struct device *dev);
 extern void pm_qos_sysfs_remove_flags(struct device *dev);
 extern int pm_qos_sysfs_add_latency_tolerance(struct device *dev);
 extern void pm_qos_sysfs_remove_latency_tolerance(struct device *dev);
+extern void sysfs_notify_runtime_status(struct device *dev);
 
 #else /* CONFIG_PM */
 
diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
index b753355..3a3e413 100644
--- a/drivers/base/power/runtime.c
+++ b/drivers/base/power/runtime.c
@@ -94,6 +94,7 @@ static void __update_runtime_status(struct device *dev, enum rpm_status status)
 {
 	update_pm_runtime_accounting(dev);
 	dev->power.runtime_status = status;
+	sysfs_notify_runtime_status(dev);
 }
 
 static u64 rpm_get_accounted_time(struct device *dev, bool suspended)
diff --git a/drivers/base/power/sysfs.c b/drivers/base/power/sysfs.c
index 1b9c281c..e86d8cb 100644
--- a/drivers/base/power/sysfs.c
+++ b/drivers/base/power/sysfs.c
@@ -734,3 +734,8 @@ void dpm_sysfs_remove(struct device *dev)
 	sysfs_unmerge_group(&dev->kobj, &pm_wakeup_attr_group);
 	sysfs_remove_group(&dev->kobj, &pm_attr_group);
 }
+
+void sysfs_notify_runtime_status(struct device *dev)
+{
+	sysfs_notify(&dev->kobj, "power", "runtime_status");
+}
-- 
2.7.4

Re: [PATCH 1/2] PM-runtime: Documentation: add runtime_status ABI document

[Dominik Brodowski]

Hi!

On Sun, Sep 01, 2019 at 11:08:51PM +0900, Akinobu Mita wrote:
> +What:		/sys/devices/.../power/runtime_status
> +Date:		April 2010
> +Contact:	Dominik Brodowski <linux@dominikbrodowski.net>

Thanks for the patch! I am not sure, however, whether I am the right contact
person here -- maybe Rafael J. Wysocki as PM maintainer?

Thanks,
	Dominik

Re: [PATCH 1/2] PM-runtime: Documentation: add runtime_status ABI document

[Rafael J. Wysocki]

On Sun, Sep 1, 2019 at 4:30 PM Dominik Brodowski
<linux@dominikbrodowski.net> wrote:
>
> Hi!
>
> On Sun, Sep 01, 2019 at 11:08:51PM +0900, Akinobu Mita wrote:
> > +What:                /sys/devices/.../power/runtime_status
> > +Date:                April 2010
> > +Contact:     Dominik Brodowski <linux@dominikbrodowski.net>
>
> Thanks for the patch! I am not sure, however, whether I am the right contact
> person here -- maybe Rafael J. Wysocki as PM maintainer?

Right, please follow the convention used for the other attributes
documented in this file.

Re: [PATCH 2/2] PM-runtime: allow userspace to monitor runtime_status changes

[Rafael J. Wysocki]

On Sun, Sep 1, 2019 at 4:09 PM Akinobu Mita <akinobu.mita@gmail.com> wrote:
>
> This enables the /sys/devices/.../power/runtime_status attribute to
> allow the user space to get notifications via poll/select when the device
> runtime PM status is changed.
>
> An example use case is to avoid unnecessary accesses for device statistics
> (e.g. diskstats for block devices) while the device is in runtime suspend
> by user space LED device actitity trigger.
>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> ---
>  Documentation/ABI/testing/sysfs-devices-power | 2 ++
>  drivers/base/power/power.h                    | 1 +
>  drivers/base/power/runtime.c                  | 1 +
>  drivers/base/power/sysfs.c                    | 5 +++++
>  4 files changed, 9 insertions(+)
>
> diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
> index 3e50536..47dc357 100644
> --- a/Documentation/ABI/testing/sysfs-devices-power
> +++ b/Documentation/ABI/testing/sysfs-devices-power
> @@ -269,3 +269,5 @@ Description:
>                 the current runtime PM status of the device, which may be
>                 "suspended", "suspending", "resuming", "active", "error" (fatal
>                 error), or "unsupported" (runtime PM is disabled).
> +               This attribute allows the user space to get notifications via
> +               poll/select when the device runtime PM status is changed.
> diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
> index ec33fbdb..8891bf4 100644
> --- a/drivers/base/power/power.h
> +++ b/drivers/base/power/power.h
> @@ -74,6 +74,7 @@ extern int pm_qos_sysfs_add_flags(struct device *dev);
>  extern void pm_qos_sysfs_remove_flags(struct device *dev);
>  extern int pm_qos_sysfs_add_latency_tolerance(struct device *dev);
>  extern void pm_qos_sysfs_remove_latency_tolerance(struct device *dev);
> +extern void sysfs_notify_runtime_status(struct device *dev);
>
>  #else /* CONFIG_PM */
>
> diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> index b753355..3a3e413 100644
> --- a/drivers/base/power/runtime.c
> +++ b/drivers/base/power/runtime.c
> @@ -94,6 +94,7 @@ static void __update_runtime_status(struct device *dev, enum rpm_status status)
>  {
>         update_pm_runtime_accounting(dev);
>         dev->power.runtime_status = status;
> +       sysfs_notify_runtime_status(dev);

There are concerns about this.

First off, it adds overhead for devices that change the PM-runtime
status relatively often.  I'm not sure if that's sufficiently
justified.

Second, it is called for status changes from "active" to "suspending"
and from "suspending" to "suspended" (and analogously for resume)
which may not be particularly useful.  At least, user space may not
have enough time to act on such notifications.

Finally, it is racy, because at the time user space does something on
a device PM-runtime status change, it very well may have changed the
other way around already.

>  }
>
>  static u64 rpm_get_accounted_time(struct device *dev, bool suspended)
> diff --git a/drivers/base/power/sysfs.c b/drivers/base/power/sysfs.c
> index 1b9c281c..e86d8cb 100644
> --- a/drivers/base/power/sysfs.c
> +++ b/drivers/base/power/sysfs.c
> @@ -734,3 +734,8 @@ void dpm_sysfs_remove(struct device *dev)
>         sysfs_unmerge_group(&dev->kobj, &pm_wakeup_attr_group);
>         sysfs_remove_group(&dev->kobj, &pm_attr_group);
>  }
> +
> +void sysfs_notify_runtime_status(struct device *dev)
> +{
> +       sysfs_notify(&dev->kobj, "power", "runtime_status");
> +}
> --
> 2.7.4
>

Re: [PATCH 2/2] PM-runtime: allow userspace to monitor runtime_status changes

[Akinobu Mita]

2019年9月3日(火) 6:47 Rafael J. Wysocki <rafael@kernel.org>:
>
> On Sun, Sep 1, 2019 at 4:09 PM Akinobu Mita <akinobu.mita@gmail.com> wrote:
> >
> > This enables the /sys/devices/.../power/runtime_status attribute to
> > allow the user space to get notifications via poll/select when the device
> > runtime PM status is changed.
> >
> > An example use case is to avoid unnecessary accesses for device statistics
> > (e.g. diskstats for block devices) while the device is in runtime suspend
> > by user space LED device actitity trigger.
> >
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> > ---
> >  Documentation/ABI/testing/sysfs-devices-power | 2 ++
> >  drivers/base/power/power.h                    | 1 +
> >  drivers/base/power/runtime.c                  | 1 +
> >  drivers/base/power/sysfs.c                    | 5 +++++
> >  4 files changed, 9 insertions(+)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
> > index 3e50536..47dc357 100644
> > --- a/Documentation/ABI/testing/sysfs-devices-power
> > +++ b/Documentation/ABI/testing/sysfs-devices-power
> > @@ -269,3 +269,5 @@ Description:
> >                 the current runtime PM status of the device, which may be
> >                 "suspended", "suspending", "resuming", "active", "error" (fatal
> >                 error), or "unsupported" (runtime PM is disabled).
> > +               This attribute allows the user space to get notifications via
> > +               poll/select when the device runtime PM status is changed.
> > diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
> > index ec33fbdb..8891bf4 100644
> > --- a/drivers/base/power/power.h
> > +++ b/drivers/base/power/power.h
> > @@ -74,6 +74,7 @@ extern int pm_qos_sysfs_add_flags(struct device *dev);
> >  extern void pm_qos_sysfs_remove_flags(struct device *dev);
> >  extern int pm_qos_sysfs_add_latency_tolerance(struct device *dev);
> >  extern void pm_qos_sysfs_remove_latency_tolerance(struct device *dev);
> > +extern void sysfs_notify_runtime_status(struct device *dev);
> >
> >  #else /* CONFIG_PM */
> >
> > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > index b753355..3a3e413 100644
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -94,6 +94,7 @@ static void __update_runtime_status(struct device *dev, enum rpm_status status)
> >  {
> >         update_pm_runtime_accounting(dev);
> >         dev->power.runtime_status = status;
> > +       sysfs_notify_runtime_status(dev);
>
> There are concerns about this.
>
> First off, it adds overhead for devices that change the PM-runtime
> status relatively often.  I'm not sure if that's sufficiently
> justified.
>
> Second, it is called for status changes from "active" to "suspending"
> and from "suspending" to "suspended" (and analogously for resume)
> which may not be particularly useful.  At least, user space may not
> have enough time to act on such notifications.
>
> Finally, it is racy, because at the time user space does something on
> a device PM-runtime status change, it very well may have changed the
> other way around already.

I withdraw this patch now.  I hope I'll retry with a real use case example
program.

Re: [PATCH 2/2] PM-runtime: allow userspace to monitor runtime_status changes

[Alan Stern]

On Tue, 3 Sep 2019, Akinobu Mita wrote:

> 2019年9月3日(火) 6:47 Rafael J. Wysocki <rafael@kernel.org>:
> >
> > On Sun, Sep 1, 2019 at 4:09 PM Akinobu Mita <akinobu.mita@gmail.com> wrote:
> > >
> > > This enables the /sys/devices/.../power/runtime_status attribute to
> > > allow the user space to get notifications via poll/select when the device
> > > runtime PM status is changed.
> > >
> > > An example use case is to avoid unnecessary accesses for device statistics
> > > (e.g. diskstats for block devices) while the device is in runtime suspend
> > > by user space LED device actitity trigger.
> > >
> > > Cc: Alan Stern <stern@rowland.harvard.edu>
> > > Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> > > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > > Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>

> > There are concerns about this.
> >
> > First off, it adds overhead for devices that change the PM-runtime
> > status relatively often.  I'm not sure if that's sufficiently
> > justified.
> >
> > Second, it is called for status changes from "active" to "suspending"
> > and from "suspending" to "suspended" (and analogously for resume)
> > which may not be particularly useful.  At least, user space may not
> > have enough time to act on such notifications.
> >
> > Finally, it is racy, because at the time user space does something on
> > a device PM-runtime status change, it very well may have changed the
> > other way around already.
> 
> I withdraw this patch now.  I hope I'll retry with a real use case example
> program.

You might want to take a look at commit 7794f486ed0b ("usbfs: Add
ioctls for runtime power management") in the usb-next branch of Greg
KH's usb.git tree.  It adds a mechanism for user programs to wait until
a device has done a runtime resume, without races.

But it is not general purpose (it applies only to USB devices), it
doesn't use poll(), and it doesn't check for other kinds of runtime PM
status changes.

Alan Stern