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Message-ID: <d4e9bf6c-0a71-49d1-a74b-76eba3af6a51@acm.org>
Date: Thu, 21 Dec 2023 11:25:47 -0800
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Subject: Re: [PATCH RFC v3 11/11] scsi: ufs: core: Perform read back before
 writing run/stop regs
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To: Andrew Halaney <ahalaney@redhat.com>, Andy Gross <agross@kernel.org>,
 Bjorn Andersson <andersson@kernel.org>,
 Konrad Dybcio <konrad.dybcio@linaro.org>,
 Manivannan Sadhasivam <mani@kernel.org>,
 "James E.J. Bottomley" <jejb@linux.ibm.com>,
 "Martin K. Petersen" <martin.petersen@oracle.com>,
 Hannes Reinecke <hare@suse.de>, Janek Kotas <jank@cadence.com>,
 Alim Akhtar <alim.akhtar@samsung.com>, Avri Altman <avri.altman@wdc.com>,
 Can Guo <quic_cang@quicinc.com>
Cc: Will Deacon <will@kernel.org>, linux-arm-msm@vger.kernel.org,
 linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20231221-ufs-reset-ensure-effect-before-delay-v3-0-2195a1b66d2e@redhat.com>
 <20231221-ufs-reset-ensure-effect-before-delay-v3-11-2195a1b66d2e@redhat.com>
From: Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <20231221-ufs-reset-ensure-effect-before-delay-v3-11-2195a1b66d2e@redhat.com>
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On 12/21/23 11:09, Andrew Halaney wrote:
> Currently a wmb() is used to ensure that writes to the
> UTP_TASK_REQ_LIST_BASE* regs are completed prior to following writes to
> the run/stop registers.
> 
> wmb() ensure that the write completes, but completion doesn't mean that
> it isn't stored in a buffer somewhere. The recommendation for
> ensuring the 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 wmb()'s
> purpose wasn't to add extra ordering (on top of the ordering guaranteed
> by writel()/readl()), it can safely be removed.

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