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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 v4 06/11] scsi: ufs: cdns-pltfrm: Perform read back
 after writing HCLKDIV
Date: Mon, 22 Jan 2024 11:24:02 -0600
Message-ID: <20240122-ufs-reset-ensure-effect-before-delay-v4-6-90a54c832508@redhat.com>
In-Reply-To: <20240122-ufs-reset-ensure-effect-before-delay-v4-0-90a54c832508@redhat.com>
References: <20240122-ufs-reset-ensure-effect-before-delay-v4-0-90a54c832508@redhat.com>
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Currently, HCLKDIV is written to and then completed with an mb().=0D
=0D
mb() ensure that the write completes, but completion doesn't mean that=0D
it isn't stored in a buffer somewhere. The recommendation for=0D
ensuring this bit has taken effect on the device is to perform a read=0D
back to force it to make it all the way to the device. This is=0D
documented in device-io.rst and a talk by Will Deacon on this can=0D
be seen over here:=0D
=0D
    https://youtu.be/i6DayghhA8Q?si=3DMiyxB5cKJXSaoc01&t=3D1678=0D
=0D
Let's do that to ensure the bit hits the device. Because the mb()'s=0D
purpose wasn't to add extra ordering (on top of the ordering guaranteed=0D
by writel()/readl()), it can safely be removed.=0D
=0D
Fixes: d90996dae8e4 ("scsi: ufs: Add UFS platform driver for Cadence UFS")=
=0D
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>=0D
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>=0D
---=0D
 drivers/ufs/host/cdns-pltfrm.c | 2 +-=0D
 1 file changed, 1 insertion(+), 1 deletion(-)=0D
=0D
diff --git a/drivers/ufs/host/cdns-pltfrm.c b/drivers/ufs/host/cdns-pltfrm.=
c=0D
index bb30267da471..66811d8d1929 100644=0D
--- a/drivers/ufs/host/cdns-pltfrm.c=0D
+++ b/drivers/ufs/host/cdns-pltfrm.c=0D
@@ -136,7 +136,7 @@ static int cdns_ufs_set_hclkdiv(struct ufs_hba *hba)=0D
 	 * Make sure the register was updated,=0D
 	 * UniPro layer will not work with an incorrect value.=0D
 	 */=0D
-	mb();=0D
+	ufshcd_readl(hba, CDNS_UFS_REG_HCLKDIV);=0D
 =0D
 	return 0;=0D
 }=0D
=0D
-- =0D
2.43.0=0D
=0D