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

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Message-ID: <20240402044900.GE2933@thinkpad>
Date: Tue, 2 Apr 2024 10:19:00 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Andrew Halaney <ahalaney@...hat.com>
Cc: Andy Gross <agross@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.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>, Will Deacon <will@...nel.org>,
	linux-arm-msm@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 10/11] scsi: ufs: core: Remove unnecessary wmb() after
 ringing doorbell

On Fri, Mar 29, 2024 at 03:46:52PM -0500, Andrew Halaney wrote:
> 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>

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>

- Mani

> ---
>  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
> 
> 

-- 
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