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From: Andrew Halaney <ahalaney@redhat.com>
To: Andy Gross <agross@kernel.org>,
	Bjorn Andersson <andersson@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	Manivannan Sadhasivam <mani@kernel.org>,
	"James E.J. Bottomley" <jejb@linux.ibm.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Hannes Reinecke <hare@suse.de>,
	Janek Kotas <jank@cadence.com>,
	Alim Akhtar <alim.akhtar@samsung.com>,
	Avri Altman <avri.altman@wdc.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Can Guo <quic_cang@quicinc.com>
Cc: Andrew Halaney <ahalaney@redhat.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-msm@vger.kernel.org,
	linux-scsi@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Subject: [PATCH RFC v3 01/11] scsi: ufs: qcom: Perform read back after writing reset bit
Date: Thu, 21 Dec 2023 13:09:47 -0600
Message-ID: <20231221-ufs-reset-ensure-effect-before-delay-v3-1-2195a1b66d2e@redhat.com>
In-Reply-To: <20231221-ufs-reset-ensure-effect-before-delay-v3-0-2195a1b66d2e@redhat.com>
References: <20231221-ufs-reset-ensure-effect-before-delay-v3-0-2195a1b66d2e@redhat.com>
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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() ensure 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>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
---
 drivers/ufs/host/ufs-qcom.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/ufs/host/ufs-qcom.h b/drivers/ufs/host/ufs-qcom.h
index 9dd9a391ebb7..b9de170983c9 100644
--- 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 */

-- 
2.43.0