scsi: ufs: qcom: Perform read back after writing unipro mode

[ Upstream commit 823150ecf0 ]

Currently, the QUNIPRO_SEL bit is written to 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

But, there's really no reason to even ensure completion before
continuing. The only requirement here is that this write is ordered to this
endpoint (which readl()/writel() guarantees already). For that reason the
mb() can be dropped altogether without anything forcing completion.

Fixes: f06fcc7155 ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20240329-ufs-reset-ensure-effect-before-delay-v5-4-181252004586@redhat.com
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/ufs/host/ufs-qcom.c

@@ -284,9 +284,6 @@ static void ufs_qcom_select_unipro_mode(struct ufs_qcom_host *host)
 
 	if (host->hw_ver.major >= 0x05)
 		ufshcd_rmwl(host->hba, QUNIPRO_G4_SEL, 0, REG_UFS_CFG0);
-
-	/* make sure above configuration is applied before we return */
-	mb();
 }
 
 /*