linux-kernel - [PATCH RFC v3 06/11] scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV

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Message-ID: <20231221-ufs-reset-ensure-effect-before-delay-v3-6-2195a1b66d2e@redhat.com>
Date: Thu, 21 Dec 2023 13:09:52 -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 06/11] scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV

Currently, HCLKDIV is written to and then completed with an mb().

mb() ensure that the write completes, but completion doesn't mean that
it isn't stored in a buffer somewhere. The recommendation for
ensuring this bit has 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 bit hits the device. Because the mb()'s
purpose wasn't to add extra ordering (on top of the ordering guaranteed
by writel()/readl()), it can safely be removed.

Fixes: d90996dae8e4 ("scsi: ufs: Add UFS platform driver for Cadence UFS")
Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/ufs/host/cdns-pltfrm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/ufs/host/cdns-pltfrm.c b/drivers/ufs/host/cdns-pltfrm.c
index bb30267da471..66811d8d1929 100644
--- a/drivers/ufs/host/cdns-pltfrm.c
+++ b/drivers/ufs/host/cdns-pltfrm.c
@@ -136,7 +136,7 @@ static int cdns_ufs_set_hclkdiv(struct ufs_hba *hba)
 	 * Make sure the register was updated,
 	 * UniPro layer will not work with an incorrect value.
 	 */
-	mb();
+	ufshcd_readl(hba, CDNS_UFS_REG_HCLKDIV);
 
 	return 0;
 }

-- 
2.43.0