scsi: ufs: qcom: Perform read back after writing CGC enable

[ Upstream commit d9488511b3 ]

Currently, the CGC enable bit is written and then an mb() is used to ensure
that completes before continuing.

mb() ensures that the write completes, but completion doesn't mean that it
isn't stored in a buffer somewhere. The recommendation for ensuring this
bit has taken effect on the device is to perform a read back to force it to
make it all the way to the device. This is documented in device-io.rst and
a talk by Will Deacon on this can be seen over here:

    https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678

Let's do that to ensure the bit hits the device. Because the mb()'s purpose
wasn't to add extra ordering (on top of the ordering guaranteed by
writel()/readl()), it can safely be removed.

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Can Guo <quic_cang@quicinc.com>
Fixes: 81c0fc51b7 ("ufs-qcom: add support for Qualcomm Technologies Inc platforms")
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20240329-ufs-reset-ensure-effect-before-delay-v5-5-181252004586@redhat.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/scsi/ufs/ufs-qcom.c

@@ -353,7 +353,7 @@ static void ufs_qcom_enable_hw_clk_gating(struct ufs_hba *hba)
 		REG_UFS_CFG2);
 
 	/* Ensure that HW clock gating is enabled before next operations */
-	mb();
+	ufshcd_readl(hba, REG_UFS_CFG2);
 }
 
 static int ufs_qcom_hce_enable_notify(struct ufs_hba *hba,