[PATCH V1] ufs: ufs-qcom: Fix UFS OCP issue during UFS power down(PC=3)

[Nitin Rawat <nitin.rawat@oss.qualcomm.com>]

According to UFS specifications, the power-off sequence for a UFS
device includes:

- Sending an SSU command with Power_Condition=3 and await a
  response.
- Asserting RST_N low.
- Turning off REF_CLK.
- Turning off VCC.
- Turning off VCCQ/VCCQ2.

As part of ufs shutdown , after the SSU command completion, asserting
hardware reset (HWRST) triggers the device firmware to wake up and
execute its reset routine. This routine initializes hardware blocks
and takes a few milliseconds to complete. During this time, the
ICCQ draws a large current.

This large ICCQ current may cause issues for the regulator which
is supplying power to UFS, because the turn off request from UFS
driver to the regulator framework will be immediately followed by
low power mode(LPM) request by regulator framework. This is done
by framework because UFS which is the only client is requesting
for disable. So if the rail is still in the process of shutting down
while ICCQ exceeds LPM current thresholds, and LPM mode is activated
in hardware during this state, it may trigger an overcurrent
protection (OCP) fault in the regulator.

To prevent this, a 10ms delay is added after asserting HWRST. This
allows the reset operation to complete while power rails remain active
and in high-power mode.

Currently there is no way for Host to query whether the reset is
completed or not and hence this the delay is based on experiments
with Qualcomm UFS controllers across multiple UFS vendors.

Signed-off-by: Nitin Rawat <nitin.rawat@oss.qualcomm.com>
---
 drivers/ufs/host/ufs-qcom.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
index 89a3328a7a75..cb54628be466 100644
--- a/drivers/ufs/host/ufs-qcom.c
+++ b/drivers/ufs/host/ufs-qcom.c
@@ -744,8 +744,21 @@ static int ufs_qcom_suspend(struct ufs_hba *hba, enum ufs_pm_op pm_op,


 	/* reset the connected UFS device during power down */
-	if (ufs_qcom_is_link_off(hba) && host->device_reset)
+	if (ufs_qcom_is_link_off(hba) && host->device_reset) {
 		ufs_qcom_device_reset_ctrl(hba, true);
+		/*
+		 * After sending the SSU command, asserting the rst_n
+		 * line causes the device firmware to wake up and
+		 * execute its reset routine.
+		 *
+		 * During this process, the device may draw current
+		 * beyond the permissible limit for low-power mode (LPM).
+		 * A 10ms delay, based on experimental observations,
+		 * allows the UFS device to complete its hardware reset
+		 * before transitioning the power rail to LPM.
+		 */
+		usleep_range(10000, 11000);
+	}

 	return ufs_qcom_ice_suspend(host);
 }
--
2.50.1