Re: [PATCH RFC v3 10/11] scsi: ufs: core: Perform read back to commit doorbell

[Manivannan Sadhasivam]

On Thu, Dec 21, 2023 at 01:09:56PM -0600, Andrew Halaney wrote:
> Currently, the doorbell is written to and a wmb() is used to commit it
> immediately.
> 
> wmb() ensures that the write completes before following writes occur,
> but completion doesn't mean that it isn't stored in a buffer somewhere.
> The recommendation for ensuring this bit has 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
> 
> Let's do that to ensure the bit hits the device. Because the wmb()'s
> purpose wasn't to add extra ordering (on top of the ordering guaranteed
> by writel()/readl()), it can safely be removed.
> 
> Fixes: ad1a1b9cd67a ("scsi: ufs: commit descriptors before setting the doorbell")
> Signed-off-by: Andrew Halaney <ahalaney@xxxxxxxxxx>

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

- Mani

> ---
>  drivers/ufs/core/ufshcd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> index 75a03ee9a1ba..caebd589e08c 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -7050,7 +7050,7 @@ static int __ufshcd_issue_tm_cmd(struct ufs_hba *hba,
>  
>  	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TASK_REQ_DOOR_BELL);
>  	/* Make sure that doorbell is committed immediately */
> -	wmb();
> +	ufshcd_readl(hba, REG_UTP_TASK_REQ_DOOR_BELL);
>  
>  	spin_unlock_irqrestore(host->host_lock, flags);
>  
> 
> -- 
> 2.43.0
> 

-- 
மணிவண்ணன் சதாசிவம்