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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>,
	Anjana Hari <quic_ahari@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
Subject: [PATCH v5 04/11] scsi: ufs: qcom: Perform read back after writing
 unipro mode
Date: Fri, 29 Mar 2024 15:46:46 -0500
Message-ID: <20240329-ufs-reset-ensure-effect-before-delay-v5-4-181252004586@redhat.com>
In-Reply-To: <20240329-ufs-reset-ensure-effect-before-delay-v5-0-181252004586@redhat.com>
References: <20240329-ufs-reset-ensure-effect-before-delay-v5-0-181252004586@redhat.com>
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Currently, the QUNIPRO_SEL bit is written to and then an mb() is used to=0D
ensure that completes before continuing.=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
But, there's really no reason to even ensure completion before=0D
continuing. The only requirement here is that this write is ordered to=0D
this endpoint (which readl()/writel() guarantees already). For that=0D
reason the mb() can be dropped altogether without anything forcing=0D
completion.=0D
=0D
Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and powe=
r optimizations")=0D
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>=0D
---=0D
 drivers/ufs/host/ufs-qcom.c | 3 ---=0D
 1 file changed, 3 deletions(-)=0D
=0D
diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c=0D
index 649fada24345..66a6c95f5d72 100644=0D
--- a/drivers/ufs/host/ufs-qcom.c=0D
+++ b/drivers/ufs/host/ufs-qcom.c=0D
@@ -278,9 +278,6 @@ static void ufs_qcom_select_unipro_mode(struct ufs_qcom=
_host *host)=0D
 =0D
 	if (host->hw_ver.major >=3D 0x05)=0D
 		ufshcd_rmwl(host->hba, QUNIPRO_G4_SEL, 0, REG_UFS_CFG0);=0D
-=0D
-	/* make sure above configuration is applied before we return */=0D
-	mb();=0D
 }=0D
 =0D
 /*=0D
=0D
-- =0D
2.44.0=0D
=0D