[PATCH RFC v2 06/11] scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV

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>,
	 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>
Subject: [PATCH RFC v2 06/11] scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV
Date: Thu, 21 Dec 2023 12:25:23 -0600	[thread overview]
Message-ID: <20231221-ufs-reset-ensure-effect-before-delay-v2-6-75af2a9bae51@redhat.com> (raw)
In-Reply-To: <20231221-ufs-reset-ensure-effect-before-delay-v2-0-75af2a9bae51@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

