linux-kernel - [PATCH RFC v4 11/11] scsi: ufs: core: Remove unnecessary wmb() prior to writing run/stop regs

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Message-ID: <20240122-ufs-reset-ensure-effect-before-delay-v4-11-90a54c832508@redhat.com>
Date: Mon, 22 Jan 2024 11:24:07 -0600
From: Andrew Halaney <ahalaney@...hat.com>
To: Andy Gross <agross@...nel.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Manivannan Sadhasivam <mani@...nel.org>,
	"James E.J. Bottomley" <jejb@...ux.ibm.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Hannes Reinecke <hare@...e.de>,
	Janek Kotas <jank@...ence.com>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Avri Altman <avri.altman@....com>,
	Bart Van Assche <bvanassche@....org>,
	Can Guo <quic_cang@...cinc.com>
Cc: Andrew Halaney <ahalaney@...hat.com>,
	Will Deacon <will@...nel.org>,
	linux-arm-msm@...r.kernel.org,
	linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH RFC v4 11/11] scsi: ufs: core: Remove unnecessary wmb()
 prior to writing run/stop regs

Currently a wmb() is used to ensure that writes to the
UTP_TASK_REQ_LIST_BASE* regs are completed prior to following writes to
the run/stop registers.

wmb() ensure that the write completes, but completion doesn't mean that
it isn't stored in a buffer somewhere. The recommendation for
ensuring the bits have 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

But, none of that is necessary here. All of the writel()/readl()'s here
are to the same endpoint, so they will be ordered. There's no subsequent
delay() etc that requires it to have taken effect already, so no
readback is necessary here.

For that reason just drop the wmb() altogether.

Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")
Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/ufs/core/ufshcd.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index 21f93d8e5818..358857ea9ff6 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -4722,12 +4722,6 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)
 	ufshcd_writel(hba, upper_32_bits(hba->utmrdl_dma_addr),
 			REG_UTP_TASK_REQ_LIST_BASE_H);

-	/*
-	 * Make sure base address and interrupt setup are updated before
-	 * enabling the run/stop registers below.
-	 */
-	wmb();
-
 	/*
 	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1
 	 */

-- 
2.43.0