linux-kernel - Re: [PATCH RFC v2 00/11] scsi: ufs: Remove overzealous memory barriers

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Date: Thu, 21 Dec 2023 12:31:23 -0600
From: Andrew Halaney <ahalaney@...hat.com>
To: 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>, 
	Yaniv Gardi <ygardi@...eaurora.org>, Dov Levenglick <dovl@...eaurora.org>, 
	Hannes Reinecke <hare@...e.de>, Subhash Jadavani <subhashj@...eaurora.org>, 
	Gilad Broner <gbroner@...eaurora.org>, Venkat Gopalakrishnan <venkatg@...eaurora.org>, 
	Janek Kotas <jank@...ence.com>, Alim Akhtar <alim.akhtar@...sung.com>, 
	Avri Altman <avri.altman@....com>, Bart Van Assche <bvanassche@....org>, 
	Anjana Hari <quic_ahari@...cinc.com>, Dolev Raviv <draviv@...eaurora.org>, 
	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 v2 00/11] scsi: ufs: Remove overzealous memory
 barriers

Sorry for the spam, b4 and smtp with google are not playing nice and
failed (but worked fine with --reflect). I'll send a v3 the old school way.

My apologies,
Andrew

On Thu, Dec 21, 2023 at 12:25:17PM -0600, Andrew Halaney wrote:
> This is an RFC because I'm not all the confident in this topic. UFS has
> a lot of mb() variants used, most with comments saying "ensure this
> takes effect before continuing". mb()'s aren't really the way to
> guarantee that, a read back is the best method.
> 
> Some of these though I think could go a step further and remove the mb()
> variant without a read back. As far as I can tell there's no real reason
> to ensure it takes effect in most cases (there's no delay() or anything
> afterwards, and eventually another readl()/writel() happens which is by
> definition ordered).
> 
> In this current series I don't do that as I wasn't totally convinced,
> but it should be considered when reviewing.
> 
> Hopefully this series gets enough interest where we can confidently
> merge the final result after review helps improve it.
> 
> I'll be travelling a good bit the next 2ish weeks, so expect little
> response until my return.
> 
> Thanks in advance for the help,
> Andrew
> 
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
> ---
> Changes in v2:
> - Added review tags for original patch
> - Added new patches to address all other memory barriers used
> 
> - Link to v1: https://lore.kernel.org/r/20231208-ufs-reset-ensure-effect-before-delay-v1-1-8a0f82d7a09e@redhat.com
> 
> ---
> Andrew Halaney (11):
>       scsi: ufs: qcom: Perform read back after writing reset bit
>       scsi: ufs: qcom: Perform read back after writing REG_UFS_SYS1CLK_1US
>       scsi: ufs: qcom: Perform read back after writing testbus config
>       scsi: ufs: qcom: Perform read back after writing unipro mode
>       scsi: ufs: qcom: Perform read back after writing CGC enable
>       scsi: ufs: cdns-pltfrm: Perform read back after writing HCLKDIV
>       scsi: ufs: core: Perform read back after writing UTP_TASK_REQ_LIST_BASE_H
>       scsi: ufs: core: Perform read back after disabling interrupts
>       scsi: ufs: core: Perform read back after disabling UIC_COMMAND_COMPL
>       scsi: ufs: core: Perform read back to commit doorbell
>       scsi: ufs: core: Perform read back before writing run/stop regs
> 
>  drivers/ufs/core/ufshcd.c      | 10 +++++-----
>  drivers/ufs/host/cdns-pltfrm.c |  2 +-
>  drivers/ufs/host/ufs-qcom.c    | 14 ++++++--------
>  drivers/ufs/host/ufs-qcom.h    | 12 ++++++------
>  4 files changed, 18 insertions(+), 20 deletions(-)
> ---
> base-commit: 20d857259d7d10cd0d5e8b60608455986167cfad
> change-id: 20231208-ufs-reset-ensure-effect-before-delay-6e06899d5419
> 
> Best regards,
> -- 
> Andrew Halaney <ahalaney@...hat.com>
>