linux-kernel - [PATCH v2 4/5] scsi: ufs: core: Remove redundant wmb() in ufshcd_send

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Message-Id: <20220423140245.394092-5-manivannan.sadhasivam@linaro.org>
Date:   Sat, 23 Apr 2022 19:32:44 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     martin.petersen@...cle.com, jejb@...ux.ibm.com
Cc:     avri.altman@....com, alim.akhtar@...sung.com,
        bjorn.andersson@...aro.org, linux-arm-msm@...r.kernel.org,
        quic_asutoshd@...cinc.com, quic_cang@...cinc.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        bvanassche@....org, ahalaney@...hat.com,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Subject: [PATCH v2 4/5] scsi: ufs: core: Remove redundant wmb() in ufshcd_send_command()

The wmb() inside ufshcd_send_command() is added to make sure that the
doorbell is committed immediately. This leads to couple of expectations:

1. The doorbell write should complete before the function return.
2. The doorbell write should not cross the function boundary.

2nd expectation is fullfilled by the Linux memory model as there is a
guarantee that the critical section won't cross the unlock (release)
operation.

1st expectation is not really needed here as there is no following read/
write that depends on the doorbell to be complete implicitly. Even if the
doorbell write is in a CPUs Write Buffer (WB), wmb() won't flush it. And
there is no real need of a WB flush here as well.

So let's get rid of the wmb() that seems redundant.

Reviewed-by: Bart Van Assche <bvanassche@....org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
---
 drivers/scsi/ufs/ufshcd.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index 9349557b8a01..ec514a6c5393 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -2116,9 +2116,6 @@ void ufshcd_send_command(struct ufs_hba *hba, unsigned int task_tag)
 	__set_bit(task_tag, &hba->outstanding_reqs);
 	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TRANSFER_REQ_DOOR_BELL);
 	spin_unlock_irqrestore(&hba->outstanding_lock, flags);
-
-	/* Make sure that doorbell is committed immediately */
-	wmb();
 }

 /**
-- 
2.25.1