[RFC,v2,00/11] scsi: ufs: Remove overzealous memory barriers

Message

Andrew Halaney Dec. 21, 2023, 6:25 p.m. UTC

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 <[e-mail]>
---
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/[e-mail]

---
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,

Comments

Andrew Halaney Dec. 21, 2023, 6:31 p.m. UTC | #1

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 <[e-mail]>
> ---
> 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/[e-mail]
> 
> ---
> 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 <[e-mail]>
>