Re: [PATCH RFC v2 06/11] scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV

From: Manivannan Sadhasivam <mani@kernel.org>
To: Andrew Halaney <ahalaney@redhat.com>
Cc: Andy Gross <agross@kernel.org>,
	Bjorn Andersson <andersson@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	"James E.J. Bottomley" <jejb@linux.ibm.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Yaniv Gardi <ygardi@codeaurora.org>,
	Dov Levenglick <dovl@codeaurora.org>,
	Hannes Reinecke <hare@suse.de>,
	Subhash Jadavani <subhashj@codeaurora.org>,
	Gilad Broner <gbroner@codeaurora.org>,
	Venkat Gopalakrishnan <venkatg@codeaurora.org>,
	Janek Kotas <jank@cadence.com>,
	Alim Akhtar <alim.akhtar@samsung.com>,
	Avri Altman <avri.altman@wdc.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Anjana Hari <quic_ahari@quicinc.com>,
	Dolev Raviv <draviv@codeaurora.org>,
	Can Guo <quic_cang@quicinc.com>, Will Deacon <will@kernel.org>,
	linux-arm-msm@vger.kernel.org, linux-scsi@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC v2 06/11] scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV
Date: Wed, 27 Dec 2023 11:36:07 +0530	[thread overview]
Message-ID: <20231227060607.GD2814@thinkpad> (raw)
In-Reply-To: <20231221-ufs-reset-ensure-effect-before-delay-v2-6-75af2a9bae51@redhat.com>

On Thu, Dec 21, 2023 at 12:25:23PM -0600, Andrew Halaney wrote:
> Currently, HCLKDIV is 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 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: d90996dae8e4 ("scsi: ufs: Add UFS platform driver for Cadence UFS")
> Signed-off-by: Andrew Halaney <ahalaney@redhat.com>

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

- Mani

> ---
>  drivers/ufs/host/cdns-pltfrm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ufs/host/cdns-pltfrm.c b/drivers/ufs/host/cdns-pltfrm.c
> index bb30267da471..66811d8d1929 100644
> --- a/drivers/ufs/host/cdns-pltfrm.c
> +++ b/drivers/ufs/host/cdns-pltfrm.c
> @@ -136,7 +136,7 @@ static int cdns_ufs_set_hclkdiv(struct ufs_hba *hba)
>  	 * Make sure the register was updated,
>  	 * UniPro layer will not work with an incorrect value.
>  	 */
> -	mb();
> +	ufshcd_readl(hba, CDNS_UFS_REG_HCLKDIV);
>  
>  	return 0;
>  }
> 
> -- 
> 2.43.0
> 

-- 
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