linux-kernel - [PATCH v5 10/11] scsi: ufs: core: Remove unnecessary wmb() after ringing doorbell

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Date: Fri, 29 Mar 2024 15:46:52 -0500
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>,
	Anjana Hari <quic_ahari@...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 v5 10/11] scsi: ufs: core: Remove unnecessary wmb() after
 ringing doorbell

Currently, the doorbell is written to and a wmb() is used to commit it
immediately.

wmb() ensures that the write completes before following writes occur,
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, completion and taking effect aren't necessary to guarantee here.

There's already other examples of the doorbell being rung that don't do
this. The writel() of the doorbell guarantees prior writes by this
thread (to the request being setup for example) complete prior to the
ringing of the doorbell, and the following
wait_for_completion_io_timeout() doesn't require any special memory
barriers either.

With that in mind, just remove the wmb() altogether here.

Fixes: ad1a1b9cd67a ("scsi: ufs: commit descriptors before setting the doorbell")
Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/ufs/core/ufshcd.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index dfa4f827766a..a2f2941450fd 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -7090,10 +7090,7 @@ static int __ufshcd_issue_tm_cmd(struct ufs_hba *hba,

 	/* send command to the controller */
 	__set_bit(task_tag, &hba->outstanding_tasks);
-
 	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TASK_REQ_DOOR_BELL);
-	/* Make sure that doorbell is committed immediately */
-	wmb();

 	spin_unlock_irqrestore(host->host_lock, flags);

-- 
2.44.0