diff mbox series

[v5,11/11] scsi: ufs: core: Remove unnecessary wmb() prior to writing run/stop regs

Message ID 20240329-ufs-reset-ensure-effect-before-delay-v5-11-181252004586@redhat.com (mailing list archive)
State Not Applicable
Headers show
Series scsi: ufs: Remove overzealous memory barriers | expand

Commit Message

Andrew Halaney March 29, 2024, 8:46 p.m. UTC
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

But, none of that is necessary here. All of the writel()/readl()'s here
are to the same endpoint, so they will be ordered. There's no subsequent
delay() etc that requires it to have taken effect already, so no
readback is necessary here.

For that reason just drop the wmb() altogether.

Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
---
 drivers/ufs/core/ufshcd.c | 6 ------
 1 file changed, 6 deletions(-)

Comments

Bart Van Assche March 29, 2024, 9:49 p.m. UTC | #1
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Manivannan Sadhasivam April 2, 2024, 4:49 a.m. UTC | #2
On Fri, Mar 29, 2024 at 03:46:53PM -0500, 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
> 
> But, none of that is necessary here. All of the writel()/readl()'s here
> are to the same endpoint, so they will be ordered. There's no subsequent
> delay() etc that requires it to have taken effect already, so no
> readback is necessary here.
> 
> For that reason just drop the wmb() altogether.
> 
> Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")
> Signed-off-by: Andrew Halaney <ahalaney@redhat.com>

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

- Mani

> ---
>  drivers/ufs/core/ufshcd.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> index a2f2941450fd..cf6a24e550f0 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -4769,12 +4769,6 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)
>  	ufshcd_writel(hba, upper_32_bits(hba->utmrdl_dma_addr),
>  			REG_UTP_TASK_REQ_LIST_BASE_H);
>  
> -	/*
> -	 * Make sure base address and interrupt setup are updated before
> -	 * enabling the run/stop registers below.
> -	 */
> -	wmb();
> -
>  	/*
>  	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1
>  	 */
> 
> -- 
> 2.44.0
> 
>
diff mbox series

Patch

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index a2f2941450fd..cf6a24e550f0 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -4769,12 +4769,6 @@  int ufshcd_make_hba_operational(struct ufs_hba *hba)
 	ufshcd_writel(hba, upper_32_bits(hba->utmrdl_dma_addr),
 			REG_UTP_TASK_REQ_LIST_BASE_H);
 
-	/*
-	 * Make sure base address and interrupt setup are updated before
-	 * enabling the run/stop registers below.
-	 */
-	wmb();
-
 	/*
 	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1
 	 */