linux-kernel - Re: [PATCH RFC v3 02/11] scsi: ufs: qcom: Perform read back after writing REG_UFS_SYS1CLK

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Message-ID: <0be12f26-f78c-4f3b-9560-3061b5ff9d54@quicinc.com>
Date: Fri, 22 Dec 2023 16:18:54 +0800
From: Can Guo <quic_cang@...cinc.com>
To: Andrew Halaney <ahalaney@...hat.com>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio
	<konrad.dybcio@...aro.org>,
        Manivannan Sadhasivam <mani@...nel.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>
CC: Will Deacon <will@...nel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v3 02/11] scsi: ufs: qcom: Perform read back after
 writing REG_UFS_SYS1CLK_1US



On 12/22/2023 3:09 AM, 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")
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
> ---
>   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 480787048e75..4c15c8a1d058 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;
> 
Reviewed-by: Can Guo <quic_cang@...cinc.com>