linux-kernel - Re: [PATCH v5 02/11] scsi: ufs: qcom: Perform read back after writing REG_UFS_SYS1CLK

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Date: Tue, 2 Apr 2024 10:15:29 +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 02/11] scsi: ufs: qcom: Perform read back after
 writing REG_UFS_SYS1CLK_1US

On Fri, Mar 29, 2024 at 03:46:44PM -0500, Andrew Halaney wrote:
> Currently after writing to REG_UFS_SYS1CLK_1US a mb() is used to ensure
> that write has gone through to the device.
> 
> 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
> 
> Let's do that to ensure the bit hits the device. Because the mb()'s
> purpose wasn't to add extra ordering (on top of the ordering guaranteed
> by writel()/readl()), it can safely be removed.
> 
> Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
> Reviewed-by: Can Guo <quic_cang@...cinc.com>
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>

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

- Mani

> ---
>  drivers/ufs/host/ufs-qcom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ufs/host/ufs-qcom.c b/drivers/ufs/host/ufs-qcom.c
> index 06859e17b67b..804dc8153e7b 100644
> --- a/drivers/ufs/host/ufs-qcom.c
> +++ b/drivers/ufs/host/ufs-qcom.c
> @@ -501,7 +501,7 @@ static int ufs_qcom_cfg_timers(struct ufs_hba *hba, u32 gear,
>  		 * make sure above write gets applied before we return from
>  		 * this function.
>  		 */
> -		mb();
> +		ufshcd_readl(hba, REG_UFS_SYS1CLK_1US);
>  	}
>  
>  	return 0;
> 
> -- 
> 2.44.0
> 
> 

-- 
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