Received-SPF: pass (google.com: domain of linux-kernel+bounces-8919-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
From: Andrew Halaney <ahalaney@redhat.com>
Date: Thu, 21 Dec 2023 12:25:23 -0600
Subject: [PATCH RFC v2 06/11] scsi: ufs: cdns-pltfrm: Perform read back
 after writing HCLKDIV
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Message-Id: <20231221-ufs-reset-ensure-effect-before-delay-v2-6-75af2a9bae51@redhat.com>
References: <20231221-ufs-reset-ensure-effect-before-delay-v2-0-75af2a9bae51@redhat.com>
In-Reply-To: <20231221-ufs-reset-ensure-effect-before-delay-v2-0-75af2a9bae51@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>, 
 Yaniv Gardi <ygardi@codeaurora.org>, Dov Levenglick <dovl@codeaurora.org>, 
 Hannes Reinecke <hare@suse.de>, Subhash Jadavani <subhashj@codeaurora.org>, 
 Gilad Broner <gbroner@codeaurora.org>, 
 Venkat Gopalakrishnan <venkatg@codeaurora.org>, 
 Janek Kotas <jank@cadence.com>, Alim Akhtar <alim.akhtar@samsung.com>, 
 Avri Altman <avri.altman@wdc.com>, Bart Van Assche <bvanassche@acm.org>, 
 Anjana Hari <quic_ahari@quicinc.com>, Dolev Raviv <draviv@codeaurora.org>, 
 Can Guo <quic_cang@quicinc.com>
Cc: Will Deacon <will@kernel.org>, linux-arm-msm@vger.kernel.org, 
 linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org, 
 Andrew Halaney <ahalaney@redhat.com>

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@redhat.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