linux-kernel - Re: [PATCH v5 04/11] scsi: ufs: qcom: Perform read back after writing unipro mode

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Message-ID: <20240402044735.GD2933@thinkpad>
Date: Tue, 2 Apr 2024 10:17:35 +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 04/11] scsi: ufs: qcom: Perform read back after
 writing unipro mode

On Fri, Mar 29, 2024 at 03:46:46PM -0500, Andrew Halaney wrote:
> Currently, the QUNIPRO_SEL bit is written to and then an mb() is used to
> ensure that completes before continuing.
> 
> mb() ensure that the write completes, 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, there's really no reason to even ensure completion before
> continuing. The only requirement here is that this write is ordered to
> this endpoint (which readl()/writel() guarantees already). For that
> reason the mb() can be dropped altogether without anything forcing
> completion.
> 
> Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>

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

- Mani

> ---
>  drivers/ufs/host/ufs-qcom.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
> index 649fada24345..66a6c95f5d72 100644
> --- a/drivers/ufs/host/ufs-qcom.c
> +++ b/drivers/ufs/host/ufs-qcom.c
> @@ -278,9 +278,6 @@ static void ufs_qcom_select_unipro_mode(struct ufs_qcom_host *host)
>  
>  	if (host->hw_ver.major >= 0x05)
>  		ufshcd_rmwl(host->hba, QUNIPRO_G4_SEL, 0, REG_UFS_CFG0);
> -
> -	/* make sure above configuration is applied before we return */
> -	mb();
>  }
>  
>  /*
> 
> -- 
> 2.44.0
> 
> 

-- 
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