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From: Andrew Halaney <ahalaney@redhat.com>
Date: Mon, 22 Jan 2024 11:21:28 -0600
Subject: [PATCH RFC v4 03/11] scsi: ufs: qcom: Remove unnecessary mb()
 after writing testbus config
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Message-Id: <20240122-ufs-reset-ensure-effect-before-delay-v4-3-6c48432151cc@redhat.com>
References: <20240122-ufs-reset-ensure-effect-before-delay-v4-0-6c48432151cc@redhat.com>
In-Reply-To: <20240122-ufs-reset-ensure-effect-before-delay-v4-0-6c48432151cc@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>, 
 Hannes Reinecke <hare@suse.de>, Janek Kotas <jank@cadence.com>, 
 Alim Akhtar <alim.akhtar@samsung.com>, Avri Altman <avri.altman@wdc.com>, 
 Bart Van Assche <bvanassche@acm.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>

Currently, the testbus configuration is written and 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

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: 9c46b8676271 ("scsi: ufs-qcom: dump additional testbus registers")
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
---
 drivers/ufs/host/ufs-qcom.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
index 0603a07a23a2..a489c8c6f849 100644
--- a/drivers/ufs/host/ufs-qcom.c
+++ b/drivers/ufs/host/ufs-qcom.c
@@ -1429,11 +1429,6 @@ int ufs_qcom_testbus_config(struct ufs_qcom_host *host)
 		    (u32)host->testbus.select_minor << offset,
 		    reg);
 	ufs_qcom_enable_test_bus(host);
-	/*
-	 * Make sure the test bus configuration is
-	 * committed before returning.
-	 */
-	mb();
 
 	return 0;
 }

-- 
2.43.0