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From: Andrew Halaney <ahalaney@redhat.com>
Subject: [PATCH RFC v4 00/11] scsi: ufs: Remove overzealous memory barriers
Date: Mon, 22 Jan 2024 11:21:25 -0600
Message-Id: <20240122-ufs-reset-ensure-effect-before-delay-v4-0-6c48432151cc@redhat.com>
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To: 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>, 
 Bart Van Assche <bvanassche@acm.org>, 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, 
 Andrew Halaney <ahalaney@redhat.com>, 
 Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

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). Some of the patches in this series do that if I was
confident it was safe (or a reviewer pointed out prior that they thought
it was safe to do so).

Thanks in advance for the help,
Andrew

Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
---

Changes in v4:
- Collected Reviewed-by tags
- Changed patches 3, 4, 10, and 11 to drop the read back && mb():
    - Please note all of those patches got reviewed-by tags by either
      Can, Mani, or Bart, but one of the three pointed out that they
      thought it could be dropped altogether (some of Mani's comments
      are on my foobar'ed v2). After some consideration I
      agree. Therefore I'd appreciate re-review on those patches by
      you three to make sure that's appropriate
- Link to v3: https://lore.kernel.org/r/20231221-ufs-reset-ensure-effect-before-delay-v3-0-2195a1b66d2e@redhat.com

Changes in v3:
- Nothing changed, I just failed to send with b4 (resulting in 2 half
  sent v2 series on list)
- Link to v2: https://lore.kernel.org/r/pnwsdz3i2liivjxvtfwq6tijotgh5adyqipjjb5wdvo4jpu7yv@j6fkshm5ipue

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: Remove unnecessary mb() 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: Remove unnecessary wmb() after ringing doorbell
      scsi: ufs: core: Remove unnecessary wmb() prior to writing run/stop regs

 drivers/ufs/core/ufshcd.c      | 15 +++------------
 drivers/ufs/host/cdns-pltfrm.c |  2 +-
 drivers/ufs/host/ufs-qcom.c    | 12 ++----------
 drivers/ufs/host/ufs-qcom.h    | 12 ++++++------
 4 files changed, 12 insertions(+), 29 deletions(-)
---
base-commit: 319fbd8fc6d339e0a1c7b067eed870c518a13a02
change-id: 20231208-ufs-reset-ensure-effect-before-delay-6e06899d5419

Best regards,
-- 
Andrew Halaney <ahalaney@redhat.com>