linux-kernel - Re: [PATCH 3/5] scsi: ufs: qcom: Add a readl() to make sure ref

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Date:   Fri, 22 Apr 2022 22:03:22 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        martin.petersen@...cle.com, jejb@...ux.ibm.com
Cc:     avri.altman@....com, alim.akhtar@...sung.com,
        bjorn.andersson@...aro.org, linux-arm-msm@...r.kernel.org,
        quic_asutoshd@...cinc.com, quic_cang@...cinc.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 3/5] scsi: ufs: qcom: Add a readl() to make sure ref_clk
 gets enabled

On 4/22/22 06:21, Manivannan Sadhasivam wrote:
> In ufs_qcom_dev_ref_clk_ctrl(), it was noted that the ref_clk needs to be
> stable for atleast 1us. Eventhough there is wmb() to make sure the write
                ^              ^
Some spaces are missing.

> gets "completed", there is no guarantee that the write actually reached
> the UFS device. There is a good chance that the write could be stored in
> a Write Buffer (WB). In that case, eventhough the CPU waits for 1us, the
                                          ^
missing space----------------------------

> ref_clk might not be stable for that period.
> 
> So lets do a readl() to make sure that the previous write has reached the
> UFS device before udelay().
> 
> Cc: stable@...r.kernel.org
> Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> ---
>   drivers/scsi/ufs/ufs-qcom.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/scsi/ufs/ufs-qcom.c b/drivers/scsi/ufs/ufs-qcom.c
> index 5f0a8f646eb5..5b9986c63eed 100644
> --- a/drivers/scsi/ufs/ufs-qcom.c
> +++ b/drivers/scsi/ufs/ufs-qcom.c
> @@ -690,6 +690,12 @@ static void ufs_qcom_dev_ref_clk_ctrl(struct ufs_qcom_host *host, bool enable)
>   		/* ensure that ref_clk is enabled/disabled before we return */
>   		wmb();
>   
> +		/*
> +		 * Make sure the write to ref_clk reaches the destination and
> +		 * not stored in a Write Buffer (WB).
> +		 */
> +		readl(host->dev_ref_clk_ctrl_mmio);
> +
>   		/*
>   		 * If we call hibern8 exit after this, we need to make sure that
>   		 * device ref_clk is stable for at least 1us before the hibern8

The comment above the wmb() call looks wrong to me. How about removing 
that wmb() call?

Thanks,

Bart.