Re: [patch 3/3] Enable Aggressive Link Power management for AHCI controllers.

[Jens Axboe]

On Wed, Jun 20 2007, Kristen Carlson Accardi wrote:
> Enable Aggressive Link Power management for AHCI controllers.
> 
> This patch will set the correct bits to turn on Aggressive
> Link Power Management (ALPM) for the ahci driver.  This
> will cause the controller and disk to negotiate a lower
> power state for the link when there is no activity (see
> the AHCI 1.x spec for details).  This feature is mutually
> exclusive with Hot Plug, so when ALPM is enabled, Hot Plug
> is disabled.  ALPM will be enabled by default, but it is
> settable via the scsi host syfs interface.  Possible 
> settings for this feature are:
> 
> Setting		Effect
> ----------------------------------------------------------
> min_power	ALPM is enabled, and link set to enter 
> 		lowest power state (SLUMBER) when idle
> 		Hot plug not allowed.
> 
> max_performance	ALPM is disabled, Hot Plug is allowed
> 
> medium_power	ALPM is enabled, and link set to enter
> 		second lowest power state (PARTIAL) when
> 		idle.  Hot plug not allowed.
> 
> Signed-off-by:  Kristen Carlson Accardi <kristen.c.accardi@xxxxxxxxx>

A suggestion (it comes with a patch!) - default to max_power/almp off,
not min_power. For two reasons:

- There's such a big performance difference between the two, you really
  want max_power when booting.

- It's a lot better to default to no change, than default to enabling
  something new.

diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
index 841cf0a..e7a2072 100644
--- a/drivers/ata/ahci.c
+++ b/drivers/ata/ahci.c
@@ -786,8 +786,7 @@ static int ahci_disable_alpm(struct ata_port *ap)
 	return 0;
 }
 
-static int ahci_enable_alpm(struct ata_port *ap,
-	enum scsi_host_link_pm policy)
+static int ahci_enable_alpm(struct ata_port *ap, enum scsi_host_link_pm policy)
 {
 	struct ahci_host_priv *hpriv = ap->host->private_data;
 	void __iomem *port_mmio = ahci_port_base(ap);
@@ -808,19 +807,19 @@ static int ahci_enable_alpm(struct ata_port *ap,
 		return -EINVAL;
 	}
 
-	switch(policy) {
+	switch (policy) {
 	case SHOST_MAX_PERFORMANCE:
-		ahci_disable_alpm(ap);
-		ap->pm_policy = policy;
-		return 0;
 	case SHOST_NOT_AVAILABLE:
-	case SHOST_MIN_POWER:
 		/*
  		 * if we came here with SHOST_NOT_AVAILABLE,
  		 * it just means this is the first time we
- 		 * have tried to enable - so try to do
- 		 * min_power
+ 		 * have tried to enable - default to max performance,
+		 * and let the user go to lower power modes on request.
  		 */
+		ahci_disable_alpm(ap);
+		ap->pm_policy = SHOST_MAX_PERFORMANCE;
+		return 0;
+	case SHOST_MIN_POWER:
 		ap->pm_policy = SHOST_MIN_POWER;
 
 		/* configure HBA to enter SLUMBER */

-- 
Jens Axboe

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