linux-kernel - [PATCH RFC v3 05/11] scsi: ufs: qcom: Perform read back after writing CGC enable

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Message-ID: <20231221-ufs-reset-ensure-effect-before-delay-v3-5-2195a1b66d2e@redhat.com>
Date: Thu, 21 Dec 2023 13:09:51 -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: Andrew Halaney <ahalaney@...hat.com>,
	Will Deacon <will@...nel.org>,
	linux-arm-msm@...r.kernel.org,
	linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH RFC v3 05/11] scsi: ufs: qcom: Perform read back after writing CGC enable

Currently, the CGC enable bit is written 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

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: 81c0fc51b7a7 ("ufs-qcom: add support for Qualcomm Technologies Inc platforms")
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 ab1ff7432d11..3db19591d008 100644
--- a/drivers/ufs/host/ufs-qcom.c
+++ b/drivers/ufs/host/ufs-qcom.c
@@ -409,7 +409,7 @@ static void ufs_qcom_enable_hw_clk_gating(struct ufs_hba *hba)
 		    REG_UFS_CFG2);
 
 	/* Ensure that HW clock gating is enabled before next operations */
-	mb();
+	ufshcd_readl(hba, REG_UFS_CFG2);
 }
 
 static int ufs_qcom_hce_enable_notify(struct ufs_hba *hba,

-- 
2.43.0