linux-kernel - [PATCH RFC v3 11/11] scsi: ufs: core: Perform read back before writing run/stop regs

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Message-ID: <20231221-ufs-reset-ensure-effect-before-delay-v3-11-2195a1b66d2e@redhat.com>
Date: Thu, 21 Dec 2023 13:09:57 -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 v3 11/11] scsi: ufs: core: Perform read back before 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

Let's do that to ensure the bits hit the device. Because the wmb()'s
purpose wasn't to add extra ordering (on top of the ordering guaranteed
by writel()/readl()), it can safely be removed.

Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")
Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/ufs/core/ufshcd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index caebd589e08c..7c1975a1181f 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -4726,7 +4726,7 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)
 	 * Make sure base address and interrupt setup are updated before
 	 * enabling the run/stop registers below.
 	 */
-	wmb();
+	ufshcd_readl(hba, REG_UTP_TASK_REQ_LIST_BASE_H);
 
 	/*
 	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1

-- 
2.43.0