linux-kernel - [PATCH RFC v2 07/11] scsi: ufs: core: Perform read back after writing UTP_TASK_REQ_LIST_BASE

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Message-Id: <20231221-ufs-reset-ensure-effect-before-delay-v2-7-75af2a9bae51@redhat.com>
Date: Thu, 21 Dec 2023 12:25:24 -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 07/11] scsi: ufs: core: Perform read back after
 writing UTP_TASK_REQ_LIST_BASE_H

Currently, the UTP_TASK_REQ_LIST_BASE_L/UTP_TASK_REQ_LIST_BASE_H regs
are 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 these 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 mb()'s
purpose wasn't to add extra ordering (on top of the ordering guaranteed
by writel()/readl()), it can safely be removed.

Fixes: 88441a8d355d ("scsi: ufs: core: Add hibernation callbacks")
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 d1e33328ff3f..7bfb556e5b8e 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -10351,7 +10351,7 @@ int ufshcd_system_restore(struct device *dev)
 	 * are updated with the latest queue addresses. Only after
 	 * updating these addresses, we can queue the new commands.
 	 */
-	mb();
+	ufshcd_readl(hba, REG_UTP_TASK_REQ_LIST_BASE_H);

 	/* Resuming from hibernate, assume that link was OFF */
 	ufshcd_set_link_off(hba);

-- 
2.43.0