[PATCH 1/2] ata: libahci: Correct setting of DEVSLP register

[Srinivas Pandruvada]

We have seen that on some platforms, SATA device never show any DEVSLP
residency. This prevent power gating of SATA IP, which prevent system
to transition to low power mode in systems with SLP_S0 aka modern
standby systems. The PHY logic is off only in DEVSLP not in slumber.
Reference:
https://www.intel.com/content/dam/www/public/us/en/documents/datasheets
/332995-skylake-i-o-platform-datasheet-volume-1.pdf
Section 28.7.6.1

Here driver is trying to do read-modify-write the devslp register. But
not resetting the bits for which this driver will modify values (DITO,
MDAT and DETO). So simply reset those bits before updating to new values.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---
 drivers/ata/libahci.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/ata/libahci.c b/drivers/ata/libahci.c
index 965842a08743..f6795d261869 100644
--- a/drivers/ata/libahci.c
+++ b/drivers/ata/libahci.c
@@ -2159,6 +2159,8 @@ static void ahci_set_aggressive_devslp(struct ata_port *ap, bool sleep)
 		deto = 20;
 	}
 
+	/* Make dito, mdat, deto bits to 0s */
+	devslp &= ~GENMASK_ULL(24, 2);
 	devslp |= ((dito << PORT_DEVSLP_DITO_OFFSET) |
 		   (mdat << PORT_DEVSLP_MDAT_OFFSET) |
 		   (deto << PORT_DEVSLP_DETO_OFFSET) |
-- 
2.17.1