PM / QoS: Introcuce latency tolerance device PM QoS type

Add a new latency tolerance device PM QoS type to be use for
specifying active state (RPM_ACTIVE) memory access (DMA) latency
tolerance requirements for devices.  It may be used to prevent
hardware from choosing overly aggressive energy-saving operation
modes (causing too much latency to appear) for the whole platform.

This feature reqiures hardware support, so it only will be
available for devices having a new .set_latency_tolerance()
callback in struct dev_pm_info populated, in which case the
routine pointed to by it should implement whatever is necessary
to transfer the effective requirement value to the hardware.

Whenever the effective latency tolerance changes for the device,
its .set_latency_tolerance() callback will be executed and the
effective value will be passed to it.  If that value is negative,
which means that the list of latency tolerance requirements for
the device is empty, the callback is expected to switch the
underlying hardware latency tolerance control mechanism to an
autonomous mode if available.  If that value is PM_QOS_LATENCY_ANY,
in turn, and the hardware supports a special "no requirement"
setting, the callback is expected to use it.  That allows software
to prevent the hardware from automatically updating the device's
latency tolerance in response to its power state changes (e.g. during
transitions from D3cold to D0), which generally may be done in the
autonomous latency tolerance control mode.

If .set_latency_tolerance() is present for the device, a new
pm_qos_latency_tolerance_us attribute will be present in the
devivce's power directory in sysfs.  Then, user space can use
that attribute to specify its latency tolerance requirement for
the device, if any.  Writing "any" to it means "no requirement, but
do not let the hardware control latency tolerance" and writing
"auto" to it allows the hardware to be switched to the autonomous
mode if there are no other requirements from the kernel side in the
device's list.

This changeset includes a fix from Mika Westerberg.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 26 insertions, 1 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
index efe449bdf81..7dbf96b724e 100644
--- a/Documentation/ABI/testing/sysfs-devices-power
+++ b/Documentation/ABI/testing/sysfs-devices-power
@@ -187,7 +187,7 @@ 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
+What:		/sys/devices/.../power/pm_qos_resume_latency_us
 Date:		March 2012
 Contact:	Rafael J. Wysocki <rjw@rjwysocki.net>
 Description:
@@ -205,6 +205,31 @@ Description:
 		This attribute has no effect on system-wide suspend/resume and
 		hibernation.
 
+What:		/sys/devices/.../power/pm_qos_latency_tolerance_us
+Date:		January 2014
+Contact:	Rafael J. Wysocki <rjw@rjwysocki.net>
+Description:
+		The /sys/devices/.../power/pm_qos_latency_tolerance_us attribute
+		contains the PM QoS active state latency tolerance limit for the
+		given device in microseconds.  That is the maximum memory access
+		latency the device can suffer without any visible adverse
+		effects on user space functionality.  If that value is the
+		string "any", the latency does not matter to user space at all,
+		but hardware should not be allowed to set the latency tolerance
+		for the device automatically.
+
+		Reading "auto" from this file means that the maximum memory
+		access latency for the device may be determined automatically
+		by the hardware as needed.  Writing "auto" to it allows the
+		hardware to be switched to this mode if there are no other
+		latency tolerance requirements from the kernel side.
+
+		This attribute is only present if the feature controlled by it
+		is supported by the hardware.
+
+		This attribute has no effect on runtime suspend and resume of
+		devices and on system-wide suspend/resume and hibernation.
+
 What:		/sys/devices/.../power/pm_qos_no_power_off
 Date:		September 2012
 Contact:	Rafael J. Wysocki <rjw@rjwysocki.net>