linux-kernel - [PATCH RFC v4 04/11] scsi: ufs: qcom: Perform read back after writing unipro mode

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Message-Id: <20240122-ufs-reset-ensure-effect-before-delay-v4-4-6c48432151cc@redhat.com>
Date: Mon, 22 Jan 2024 11:21:29 -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: 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 v4 04/11] scsi: ufs: qcom: Perform read back after
 writing unipro mode

Currently, the QUNIPRO_SEL bit is written to and then an mb() is used to
ensure that completes before continuing.

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

But, there's really no reason to even ensure completion before
continuing. The only requirement here is that this write is ordered to
this endpoint (which readl()/writel() guarantees already). For that
reason the mb() can be dropped altogether without anything forcing
completion.

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 | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
index a489c8c6f849..decad95bd444 100644
--- a/drivers/ufs/host/ufs-qcom.c
+++ b/drivers/ufs/host/ufs-qcom.c
@@ -278,9 +278,6 @@ static void ufs_qcom_select_unipro_mode(struct ufs_qcom_host *host)

 	if (host->hw_ver.major >= 0x05)
 		ufshcd_rmwl(host->hba, QUNIPRO_G4_SEL, 0, REG_UFS_CFG0);
-
-	/* make sure above configuration is applied before we return */
-	mb();
 }

 /*

-- 
2.43.0