linux-kernel - Re: [PATCH v5 11/11] scsi: ufs: core: Remove unnecessary wmb() prior to writing run/stop regs

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Message-ID: <20240402044928.GF2933@thinkpad>
Date: Tue, 2 Apr 2024 10:19:28 +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 11/11] scsi: ufs: core: Remove unnecessary wmb() prior
 to writing run/stop regs

On Fri, Mar 29, 2024 at 03:46:53PM -0500, Andrew Halaney wrote:
> Currently a wmb() is used to ensure that writes to the
> UTP_TASK_REQ_LIST_BASE* regs are completed prior to following writes to
> the run/stop registers.
> 
> wmb() ensure that the write completes, but completion doesn't mean that
> it isn't stored in a buffer somewhere. The recommendation for
> ensuring the bits have 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, none of that is necessary here. All of the writel()/readl()'s here
> are to the same endpoint, so they will be ordered. There's no subsequent
> delay() etc that requires it to have taken effect already, so no
> readback is necessary here.
> 
> For that reason just drop the wmb() altogether.
> 
> Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>

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

- Mani

> ---
>  drivers/ufs/core/ufshcd.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> index a2f2941450fd..cf6a24e550f0 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -4769,12 +4769,6 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)
>  	ufshcd_writel(hba, upper_32_bits(hba->utmrdl_dma_addr),
>  			REG_UTP_TASK_REQ_LIST_BASE_H);
>  
> -	/*
> -	 * Make sure base address and interrupt setup are updated before
> -	 * enabling the run/stop registers below.
> -	 */
> -	wmb();
> -
>  	/*
>  	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1
>  	 */
> 
> -- 
> 2.44.0
> 
> 

-- 
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