diff mbox series

[v2,4/5] scsi: ufs: core: Remove redundant wmb() in ufshcd_send_command()

Message ID 20220423140245.394092-5-manivannan.sadhasivam@linaro.org (mailing list archive)
State Superseded
Headers show
Series Qcom UFS driver updates | expand

Commit Message

Manivannan Sadhasivam April 23, 2022, 2:02 p.m. UTC 
The wmb() inside ufshcd_send_command() is added to make sure that the
doorbell is committed immediately. This leads to couple of expectations:

1. The doorbell write should complete before the function return.
2. The doorbell write should not cross the function boundary.

2nd expectation is fullfilled by the Linux memory model as there is a
guarantee that the critical section won't cross the unlock (release)
operation.

1st expectation is not really needed here as there is no following read/
write that depends on the doorbell to be complete implicitly. Even if the
doorbell write is in a CPUs Write Buffer (WB), wmb() won't flush it. And
there is no real need of a WB flush here as well.

So let's get rid of the wmb() that seems redundant.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
---
 drivers/scsi/ufs/ufshcd.c | 3 ---
 1 file changed, 3 deletions(-)

Comments

Bjorn Andersson April 23, 2022, 3:19 p.m. UTC | #1 
On Sat 23 Apr 07:02 PDT 2022, Manivannan Sadhasivam wrote:

> The wmb() inside ufshcd_send_command() is added to make sure that the
> doorbell is committed immediately. This leads to couple of expectations:
> 
> 1. The doorbell write should complete before the function return.
> 2. The doorbell write should not cross the function boundary.
> 
> 2nd expectation is fullfilled by the Linux memory model as there is a
> guarantee that the critical section won't cross the unlock (release)
> operation.
> 
> 1st expectation is not really needed here as there is no following read/
> write that depends on the doorbell to be complete implicitly. Even if the
> doorbell write is in a CPUs Write Buffer (WB), wmb() won't flush it. And
> there is no real need of a WB flush here as well.
> 
> So let's get rid of the wmb() that seems redundant.
> 
> Reviewed-by: Bart Van Assche <bvanassche@acm.org>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Regards,
Bjorn

> ---
>  drivers/scsi/ufs/ufshcd.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index 9349557b8a01..ec514a6c5393 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -2116,9 +2116,6 @@ void ufshcd_send_command(struct ufs_hba *hba, unsigned int task_tag)
>  	__set_bit(task_tag, &hba->outstanding_reqs);
>  	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TRANSFER_REQ_DOOR_BELL);
>  	spin_unlock_irqrestore(&hba->outstanding_lock, flags);
> -
> -	/* Make sure that doorbell is committed immediately */
> -	wmb();
>  }
>  
>  /**
> -- 
> 2.25.1
>
Bean Huo April 24, 2022, 11:04 a.m. UTC | #2 
Acked-by: Bean Huo <beanhuo@micron.com>
diff mbox series

Patch

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index 9349557b8a01..ec514a6c5393 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -2116,9 +2116,6 @@  void ufshcd_send_command(struct ufs_hba *hba, unsigned int task_tag)
 	__set_bit(task_tag, &hba->outstanding_reqs);
 	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TRANSFER_REQ_DOOR_BELL);
 	spin_unlock_irqrestore(&hba->outstanding_lock, flags);
-
-	/* Make sure that doorbell is committed immediately */
-	wmb();
 }
 
 /**