ahci: Allow setting a default LPM policy for mobile chipsets

On many laptops setting a different LPM policy then unknown /
max_performance can lead to power-savings of 1.0 - 1.5 Watts (when idle).

Modern ultrabooks idle around 6W (at 50% screen brightness), 1.0 - 1.5W
is a significant chunk of this.

There are some performance / latency costs to enabling LPM by default,
so it is desirable to make it possible to set a different LPM policy
for mobile / laptop variants of chipsets / "South Bridges" vs their
desktop / server counterparts. Also enabling LPM by default is not
entirely without risk of regressions. At least min_power is known to
cause issues with some disks, including some reports of data corruption.

This commits adds a new ahci.mobile_lpm_policy kernel cmdline option,
which defaults to a new SATA_MOBILE_LPM_POLICY Kconfig option so that
Linux distributions can choose to set a LPM policy for mobile chipsets
by default.

The reason to have both a kernel cmdline option and a Kconfig default
value for it, is to allow easy overriding of the default to allow
trouble-shooting without needing to rebuild the kernel.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/Kconfig

@@ -92,6 +92,25 @@ config SATA_AHCI
 
 	  If unsure, say N.
 
+config SATA_MOBILE_LPM_POLICY
+	int "Default SATA Link Power Management policy for mobile chipsets"
+	range 0 4
+	default 0
+	depends on SATA_AHCI
+	help
+	  Select the Default SATA Link Power Management (LPM) policy to use
+	  for mobile / laptop variants of chipsets / "South Bridges".
+
+	  The value set has the following meanings:
+		0 => Keep firmware settings
+		1 => Maximum performance
+		2 => Medium power
+		3 => Medium power with Device Initiated PM enabled
+		4 => Minimum power
+
+	  Note "Minimum power" is known to cause issues, including disk
+	  corruption, with some disks and should not be used.
+
 config SATA_AHCI_PLATFORM
 	tristate "Platform AHCI SATA support"
 	help

drivers/ata/ahci.h

@@ -251,6 +251,9 @@ enum {
 	AHCI_HFLAG_YES_ALPM		= (1 << 23), /* force ALPM cap on */
 	AHCI_HFLAG_NO_WRITE_TO_RO	= (1 << 24), /* don't write to read
 							only registers */
+	AHCI_HFLAG_IS_MOBILE		= (1 << 25), /* mobile chipset, use
+							SATA_MOBILE_LPM_POLICY
+							as default lpm_policy */
 
 	/* ap->flags bits */