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, 9 insertions, 0 deletions

diff --git a/include/linux/pm_qos.h b/include/linux/pm_qos.h
index c8a541e..2e9191a 100644
--- a/include/linux/pm_qos.h
+++ b/include/linux/pm_qos.h
@@ -137,4 +137,13 @@ static inline int dev_pm_qos_add_ancestor_request(struct device *dev,
 			{ return 0; }
 #endif
 
+#ifdef CONFIG_PM_RUNTIME
+int dev_pm_qos_expose_latency_limit(struct device *dev, s32 value);
+void dev_pm_qos_hide_latency_limit(struct device *dev);
+#else
+static inline int dev_pm_qos_expose_latency_limit(struct device *dev, s32 value)
+			{ return 0; }
+static inline void dev_pm_qos_hide_latency_limit(struct device *dev) {}
+#endif
+
 #endif