Re: [PATCH RFC v3 09/11] scsi: ufs: core: Perform read back after disabling UIC_COMMAND_COMPL

[Bart Van Assche <bvanassche@acm.org>]

On 12/21/23 11:09, Andrew Halaney wrote:
> Currently, the UIC_COMMAND_COMPL interrupt is disabled and a wmb() is
> used to complete the register write before any following writes.
> 
> wmb() ensures the writes complete in that order, 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: d75f7fe495cf ("scsi: ufs: reduce the interrupts for power mode change requests")
> Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
> ---
>   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 bb603769b029..75a03ee9a1ba 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -4240,7 +4240,7 @@ static int ufshcd_uic_pwr_ctrl(struct ufs_hba *hba, struct uic_command *cmd)
>   		 * Make sure UIC command completion interrupt is disabled before
>   		 * issuing UIC command.
>   		 */
> -		wmb();
> +		ufshcd_readl(hba, REG_INTERRUPT_ENABLE);
>   		reenable_intr = true;
>   	}
>   	spin_unlock_irqrestore(hba->host->host_lock, flags);

Reviewed-by: Bart Van Assche <bvanassche@acm.org>