libata: Add new med_power_with_dipm link_power_management_policy setting

As described by Matthew Garret quite a while back:
https://mjg59.dreamwidth.org/34868.html

Intel CPUs starting with the Haswell generation need SATA links to power
down for the "package" part of the CPU to reach low power-states like
PC7 / P8 which bring a significant power-saving with them.

The default max_performance lpm policy does not allow for these high
PC states, both the medium_power and min_power policies do allow this.

The min_power policy saves significantly more power, but there are some
reports of some disks / SSDs not liking min_power leading to system
crashes and in some cases even data corruption has been reported.

Matthew has found a document documenting the default settings of
Intel's IRST Windows driver with which most laptops ship:
https://www-ssl.intel.com/content/dam/doc/reference-guide/sata-devices-implementation-recommendations.pdf

Matthew wrote a patch changing med_power to match those defaults, but
that never got anywhere as some people where reporting issues with the
patch-set that patch was a part of.

This commit is another attempt to make the default IRST driver settings
available under Linux, but instead of changing medium_power and
potentially introducing regressions, this commit adds a new
med_power_with_dipm setting which is identical to the existing
medium_power accept that it enables dipm on top, which makes it match
the Windows IRST driver settings, which should hopefully be safe to
use on most devices.

The med_power_with_dipm setting is close to min_power, except that:
a) It does not use host-initiated slumber mode (ASP not set),
   but it does allow device-initiated slumber
b) It does not enable DevSlp mode

On my T440s test laptop I get the following power savings when idle:
medium_power		0.9W
med_power_with_dipm	1.2W
min_power		1.2W
Suggested-by: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/libata-core.c

@@ -3964,6 +3964,7 @@ int sata_link_scr_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 		scontrol &= ~(0x1 << 8);
 		scontrol |= (0x6 << 8);
 		break;
+	case ATA_LPM_MED_POWER_WITH_DIPM:
 	case ATA_LPM_MIN_POWER:
 		if (ata_link_nr_enabled(link) > 0)
 			/* no restrictions on LPM transitions */

drivers/ata/libata-eh.c

@@ -3454,9 +3454,9 @@ static int ata_eh_maybe_retry_flush(struct ata_device *dev)
  *	@r_failed_dev: out parameter for failed device
  *
  *	Enable SATA Interface power management.  This will enable
- *	Device Interface Power Management (DIPM) for min_power
- * 	policy, and then call driver specific callbacks for
- *	enabling Host Initiated Power management.
+ *	Device Interface Power Management (DIPM) for min_power and
+ *	medium_power_with_dipm policies, and then call driver specific
+ *	callbacks for enabling Host Initiated Power management.
  *
  *	LOCKING:
  *	EH context.
@@ -3502,7 +3502,7 @@ static int ata_eh_set_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 			hints &= ~ATA_LPM_HIPM;
 
 		/* disable DIPM before changing link config */
-		if (policy != ATA_LPM_MIN_POWER && dipm) {
+		if (policy < ATA_LPM_MED_POWER_WITH_DIPM && dipm) {
 			err_mask = ata_dev_set_feature(dev,
 					SETFEATURES_SATA_DISABLE, SATA_DIPM);
 			if (err_mask && err_mask != AC_ERR_DEV) {
@@ -3545,7 +3545,7 @@ static int ata_eh_set_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 
 	/* host config updated, enable DIPM if transitioning to MIN_POWER */
 	ata_for_each_dev(dev, link, ENABLED) {
-		if (policy == ATA_LPM_MIN_POWER && !no_dipm &&
+		if (policy >= ATA_LPM_MED_POWER_WITH_DIPM && !no_dipm &&
 		    ata_id_has_dipm(dev->id)) {
 			err_mask = ata_dev_set_feature(dev,
 					SETFEATURES_SATA_ENABLE, SATA_DIPM);

drivers/ata/libata-scsi.c

@@ -109,6 +109,7 @@ static const char *ata_lpm_policy_names[] = {
 	[ATA_LPM_UNKNOWN]		= "max_performance",
 	[ATA_LPM_MAX_POWER]		= "max_performance",
 	[ATA_LPM_MED_POWER]		= "medium_power",
+	[ATA_LPM_MED_POWER_WITH_DIPM]	= "med_power_with_dipm",
 	[ATA_LPM_MIN_POWER]		= "min_power",
 };
 

include/linux/libata.h

@@ -522,6 +522,7 @@ enum ata_lpm_policy {
 	ATA_LPM_UNKNOWN,
 	ATA_LPM_MAX_POWER,
 	ATA_LPM_MED_POWER,
+	ATA_LPM_MED_POWER_WITH_DIPM, /* Med power + DIPM as win IRST does */
 	ATA_LPM_MIN_POWER,
 };