[PATCH] ata: ahci: Do not enable LPM if no LPM states are supported by the HBA

[Niklas Cassel <cassel@kernel.org>]

LPM consists of HIPM (host initiated power management) and DIPM
(device initiated power management).

ata_eh_set_lpm() will only enable HIPM if both the HBA and the device
supports it.

However, DIPM will be enabled as long as the device supports it.
The HBA will later reject the device's request to enter a power state
that it does not support (Slumber/Partial/DevSleep) (DevSleep is never
initiated by the device).

For a HBA that doesn't support any LPM states, simply don't set a LPM
policy such that all the HIPM/DIPM probing/enabling will be skipped.

Not enabling HIPM or DIPM in the first place is safer than relying on
the device following the AHCI specification and respecting the NAK.
(There are comments in the code that some devices misbehave when
receiving a NAK.)

Performing this check in ahci_update_initial_lpm_policy() also has the
advantage that a HBA that doesn't support any LPM states will take the
exact same code paths as a port that is external/hot plug capable.

Fixes: 7627a0edef54 ("ata: ahci: Drop low power policy board type")
Cc: stable@vger.kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
We have not received any bug reports with this.
The devices that were quirked recently all supported both Partial and
Slumber.
This is more a defensive action, as it seems unnecessary to enable DIPM
in the first place, if the HBA doesn't support any LPM states.

 drivers/ata/ahci.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
index 07d66d2c5f0d..214de08de642 100644
--- a/drivers/ata/ahci.c
+++ b/drivers/ata/ahci.c
@@ -1735,6 +1735,12 @@ static void ahci_update_initial_lpm_policy(struct ata_port *ap)
 	if (ap->pflags & ATA_PFLAG_EXTERNAL)
 		return;
 
+	/* If no LPM states are supported by the HBA, do not bother with LPM */
+	if ((ap->host->flags & ATA_HOST_NO_PART) &&
+	    (ap->host->flags & ATA_HOST_NO_SSC) &&
+	    (ap->host->flags & ATA_HOST_NO_DEVSLP))
+		return;
+
 	/* user modified policy via module param */
 	if (mobile_lpm_policy != -1) {
 		policy = mobile_lpm_policy;
-- 
2.45.1