[libata] Link power management infrastructure

Device Initiated Power Management, which is defined
in SATA 2.5 can be enabled for disks which support it.
This patch enables DIPM when the user sets the link
power management policy to "min_power".

Additionally, libata drivers can define a function
(enable_pm) that will perform hardware specific actions to
enable whatever power management policy the user set up
for Host Initiated Power management (HIPM).
This power management policy will be activated after all
disks have been enumerated and intialized.  Drivers should
also define disable_pm, which will turn off link power
management, but not change link power management policy.

Documentation/scsi/link_power_management_policy.txt has additional
information.

Signed-off-by:  Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/scsi/link_power_management_policy.txt b/Documentation/scsi/link_power_management_policy.txt
new file mode 100644
index 000000000000..d18993d01884
--- /dev/null
+++ b/Documentation/scsi/link_power_management_policy.txt
@@ -0,0 +1,19 @@
+This parameter allows the user to set the link (interface) power management.
+There are 3 possible options:
+
+Value                   Effect
+----------------------------------------------------------------------------
+min_power               Tell the controller to try to make the link use the
+                        least possible power when possible.  This may
+                        sacrifice some performance due to increased latency
+                        when coming out of lower power states.
+
+max_performance         Generally, this means no power management.  Tell
+                        the controller to have performance be a priority
+                        over power management.
+
+medium_power            Tell the controller to enter a lower power state
+                        when possible, but do not enter the lowest power
+                        state, thus improving latency over min_power setting.
+
+