linux-kernel - Re: [PATCH RFC v3 07/11] scsi: ufs: core: Perform read back after writing UTP_TASK_REQ_LIST_BASE

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Message-ID: <ca0b2a81-c0a0-4392-bcc1-5590caf699c7@acm.org>
Date: Thu, 21 Dec 2023 11:30:31 -0800
From: Bart Van Assche <bvanassche@....org>
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>,
 Can Guo <quic_cang@...cinc.com>
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 07/11] scsi: ufs: core: Perform read back after
 writing UTP_TASK_REQ_LIST_BASE_H

On 12/21/23 11:09, Andrew Halaney wrote:
> Currently, the UTP_TASK_REQ_LIST_BASE_L/UTP_TASK_REQ_LIST_BASE_H regs
> are written to and then completed with an mb().
> 
> mb() ensure that the write completes, but completion doesn't mean that
> it isn't stored in a buffer somewhere. The recommendation for
> ensuring these bits have 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 bits hit 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: 88441a8d355d ("scsi: ufs: core: Add hibernation callbacks")
> 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 d1e33328ff3f..7bfb556e5b8e 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -10351,7 +10351,7 @@ int ufshcd_system_restore(struct device *dev)
>   	 * are updated with the latest queue addresses. Only after
>   	 * updating these addresses, we can queue the new commands.
>   	 */
> -	mb();
> +	ufshcd_readl(hba, REG_UTP_TASK_REQ_LIST_BASE_H);
>   
>   	/* Resuming from hibernate, assume that link was OFF */
>   	ufshcd_set_link_off(hba);

Reviewed-by: Bart Van Assche <bvanassche@....org>