scsi: ufs: qcom: Perform read back after writing reset bit

Currently, the reset bit for the UFS provided reset controller (used by its
phy) is written to, and then a mb() happens to try and ensure that hit the
device. Immediately afterwards a usleep_range() occurs.

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. By doing so and
guaranteeing the ordering against the immediately following usleep_range(),
the mb() can safely be removed.

Fixes: 81c0fc51b7a7 ("ufs-qcom: add support for Qualcomm Technologies Inc platforms")
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Can Guo <quic_cang@quicinc.com>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Link: https://lore.kernel.org/r/20240329-ufs-reset-ensure-effect-before-delay-v5-1-181252004586@redhat.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

diff --git a/drivers/ufs/host/ufs-qcom.h b/drivers/ufs/host/ufs-qcom.h
index 9dd9a391ebb76273f9e51afcc848ee7a36426e5a..b9de170983c9e13e9687de46cb5bd30331d96eb5 100644 (file)
--- a/drivers/ufs/host/ufs-qcom.h
+++ b/drivers/ufs/host/ufs-qcom.h
@@ -151,10 +151,10 @@ static inline void ufs_qcom_assert_reset(struct ufs_hba *hba)
        ufshcd_rmwl(hba, UFS_PHY_SOFT_RESET, UFS_PHY_SOFT_RESET, REG_UFS_CFG1);
 
        /*
-        * Make sure assertion of ufs phy reset is written to
-        * register before returning
+        * Dummy read to ensure the write takes effect before doing any sort
+        * of delay
         */
-       mb();
+       ufshcd_readl(hba, REG_UFS_CFG1);
 }
 
 static inline void ufs_qcom_deassert_reset(struct ufs_hba *hba)
@@ -162,10 +162,10 @@ static inline void ufs_qcom_deassert_reset(struct ufs_hba *hba)
        ufshcd_rmwl(hba, UFS_PHY_SOFT_RESET, 0, REG_UFS_CFG1);
 
        /*
-        * Make sure de-assertion of ufs phy reset is written to
-        * register before returning
+        * Dummy read to ensure the write takes effect before doing any sort
+        * of delay
         */
-       mb();
+       ufshcd_readl(hba, REG_UFS_CFG1);
 }
 
 /* Host controller hardware version: major.minor.step */