linux-kernel - [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>
Date: Mon, 22 Jan 2024 11:21:28 -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 03/11] scsi: ufs: qcom: Remove unnecessary mb()
 after writing testbus config

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@...hat.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