linux-kernel - [PATCH RFC v2 02/11] scsi: ufs: qcom: Perform read back after writing REG_UFS_SYS1CLK

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Message-Id: <20231221-ufs-reset-ensure-effect-before-delay-v2-2-75af2a9bae51@redhat.com>
Date: Thu, 21 Dec 2023 12:25:19 -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>, 
 Yaniv Gardi <ygardi@...eaurora.org>, Dov Levenglick <dovl@...eaurora.org>, 
 Hannes Reinecke <hare@...e.de>, Subhash Jadavani <subhashj@...eaurora.org>, 
 Gilad Broner <gbroner@...eaurora.org>, 
 Venkat Gopalakrishnan <venkatg@...eaurora.org>, 
 Janek Kotas <jank@...ence.com>, Alim Akhtar <alim.akhtar@...sung.com>, 
 Avri Altman <avri.altman@....com>, Bart Van Assche <bvanassche@....org>, 
 Anjana Hari <quic_ahari@...cinc.com>, Dolev Raviv <draviv@...eaurora.org>, 
 Can Guo <quic_cang@...cinc.com>
Cc: Will Deacon <will@...nel.org>, linux-arm-msm@...r.kernel.org, 
 linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Andrew Halaney <ahalaney@...hat.com>
Subject: [PATCH RFC v2 02/11] scsi: ufs: qcom: Perform read back after
 writing REG_UFS_SYS1CLK_1US

Currently after writing to REG_UFS_SYS1CLK_1US a mb() is used to ensure
that write has gone through to the device.

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: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/ufs/host/ufs-qcom.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
index 480787048e75..4c15c8a1d058 100644
--- a/drivers/ufs/host/ufs-qcom.c
+++ b/drivers/ufs/host/ufs-qcom.c
@@ -501,7 +501,7 @@ static int ufs_qcom_cfg_timers(struct ufs_hba *hba, u32 gear,
 		 * make sure above write gets applied before we return from
 		 * this function.
 		 */
-		mb();
+		ufshcd_readl(hba, REG_UFS_SYS1CLK_1US);
 	}

 	return 0;

-- 
2.43.0