PM / QoS: Make it possible to expose PM QoS latency constraints

A runtime suspend of a device (e.g. an MMC controller) belonging to
a power domain or, in a more complicated scenario, a runtime suspend
of another device in the same power domain, may cause power to be
removed from the entire domain.  In that case, the amount of time
necessary to runtime-resume the given device (e.g. the MMC
controller) is often substantially greater than the time needed to
run its driver's runtime resume callback.  That may hurt performance
in some situations, because user data may need to wait for the
device to become operational, so we should make it possible to
prevent that from happening.

For this reason, introduce a new sysfs attribute for devices,
power/pm_qos_resume_latency_us, allowing user space to specify the
upper bound of the time necessary to bring the (runtime-suspended)
device up after the resume of it has been requested.  However, make
that attribute appear only for the devices whose drivers declare
support for it by calling the (new) dev_pm_qos_expose_latency_limit()
helper function with the appropriate initial value of the attribute.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
index 8ffbc25376a0..840f7d64d483 100644
--- a/Documentation/ABI/testing/sysfs-devices-power
+++ b/Documentation/ABI/testing/sysfs-devices-power
@@ -165,3 +165,21 @@ Description:
 
                 Not all drivers support this attribute.  If it isn't supported,
                 attempts to read or write it will yield I/O errors.
+
+What:           /sys/devices/.../power/pm_qos_latency_us
+Date:           March 2012
+Contact:        Rafael J. Wysocki <rjw@sisk.pl>
+Description:
+                The /sys/devices/.../power/pm_qos_resume_latency_us attribute
+                contains the PM QoS resume latency limit for the given device,
+                which is the maximum allowed time it can take to resume the
+                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.
+
+                Not all drivers support this attribute.  If it isn't supported,
+                it is not present.
+
+                This attribute has no effect on system-wide suspend/resume and
+                hibernation.