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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 11/11] scsi: ufs: core: Remove unnecessary wmb() prior
 to writing run/stop regs
Date: Fri, 29 Mar 2024 15:46:53 -0500
Message-ID: <20240329-ufs-reset-ensure-effect-before-delay-v5-11-181252004586@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 a wmb() is used to ensure that writes to the=0D
UTP_TASK_REQ_LIST_BASE* regs are completed prior to following writes to=0D
the run/stop registers.=0D
=0D
wmb() 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 the bits have 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, none of that is necessary here. All of the writel()/readl()'s here=0D
are to the same endpoint, so they will be ordered. There's no subsequent=0D
delay() etc that requires it to have taken effect already, so no=0D
readback is necessary here.=0D
=0D
For that reason just drop the wmb() altogether.=0D
=0D
Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")=0D
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>=0D
---=0D
 drivers/ufs/core/ufshcd.c | 6 ------=0D
 1 file changed, 6 deletions(-)=0D
=0D
diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c=0D
index a2f2941450fd..cf6a24e550f0 100644=0D
--- a/drivers/ufs/core/ufshcd.c=0D
+++ b/drivers/ufs/core/ufshcd.c=0D
@@ -4769,12 +4769,6 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)=
=0D
 	ufshcd_writel(hba, upper_32_bits(hba->utmrdl_dma_addr),=0D
 			REG_UTP_TASK_REQ_LIST_BASE_H);=0D
 =0D
-	/*=0D
-	 * Make sure base address and interrupt setup are updated before=0D
-	 * enabling the run/stop registers below.=0D
-	 */=0D
-	wmb();=0D
-=0D
 	/*=0D
 	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1=0D
 	 */=0D
=0D
-- =0D
2.44.0=0D
=0D