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

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Message-ID: <YmQZAQ9lz0jTzhUp@ripper>
Date:   Sat, 23 Apr 2022 08:19:29 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     martin.petersen@...cle.com, jejb@...ux.ibm.com,
        avri.altman@....com, alim.akhtar@...sung.com,
        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
Subject: Re: [PATCH v2 4/5] scsi: ufs: core: Remove redundant wmb() in
 ufshcd_send_command()

On Sat 23 Apr 07:02 PDT 2022, Manivannan Sadhasivam wrote:

> 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>

Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> ---
>  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
>