linux-kernel - Re: [PATCH RFC v3 10/11] scsi: ufs: core: Perform read back to commit doorbell

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Message-ID: <2b04cca6-ee41-47c3-86db-c52aa5f5405d@quicinc.com>
Date: Fri, 22 Dec 2023 16:27:15 +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 10/11] scsi: ufs: core: Perform read back to commit
 doorbell



On 12/22/2023 3:09 AM, Andrew Halaney wrote:
> Currently, the doorbell is written to and a wmb() is used to commit it
> immediately.
> 
> wmb() ensures that the write completes before following writes occur,
> 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 wmb()'s
> purpose wasn't to add extra ordering (on top of the ordering guaranteed
> by writel()/readl()), it can safely be removed.
> 
> Fixes: ad1a1b9cd67a ("scsi: ufs: commit descriptors before setting the doorbell")
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
> ---
>   drivers/ufs/core/ufshcd.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> index 75a03ee9a1ba..caebd589e08c 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -7050,7 +7050,7 @@ static int __ufshcd_issue_tm_cmd(struct ufs_hba *hba,
>   
>   	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TASK_REQ_DOOR_BELL);
>   	/* Make sure that doorbell is committed immediately */
> -	wmb();
> +	ufshcd_readl(hba, REG_UTP_TASK_REQ_DOOR_BELL);
>   
>   	spin_unlock_irqrestore(host->host_lock, flags);
>   
> 
Reviewed-by: Can Guo <quic_cang@...cinc.com>