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

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Date:   Fri, 22 Apr 2022 22:19:14 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        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
Subject: Re: [PATCH 4/5] scsi: ufs: core: Remove redundant wmb() in
 ufshcd_send_command()

On 4/22/22 06:21, Manivannan Sadhasivam wrote:
> The wmb() inside ufshcd_send_command() is added to make sure that the
> doorbell is committed immediately.

That's not the purpose of the wmb() call so I think the comment is wrong.

> 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.

I think you meant that the writel() won't cross the unlock operation?

> 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();
>   }

Anyway:

Reviewed-by: Bart Van Assche <bvanassche@....org>