Re: [PATCH RFC v3 11/11] scsi: ufs: core: Perform read back before writing run/stop regs

[Can Guo]

On 12/22/2023 3:09 AM, 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
>
> Let's do that to ensure the bits hit 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: 897efe628d7e ("scsi: ufs: add missing memory barriers")
> Signed-off-by: Andrew Halaney <ahalaney@xxxxxxxxxx>
> ---
>   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 caebd589e08c..7c1975a1181f 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -4726,7 +4726,7 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)
>   	 * Make sure base address and interrupt setup are updated before
>   	 * enabling the run/stop registers below.
>   	 */
> -	wmb();
> +	ufshcd_readl(hba, REG_UTP_TASK_REQ_LIST_BASE_H);
>   
>   	/*
>   	 * UCRDY, UTMRLDY and UTRLRDY bits must be 1
Reviewed-by: Can Guo <quic_cang@xxxxxxxxxxx>