linux-kernel - Re: [PATCH v4 1/7] scsi: hpsa: Eliminate duplicate barriers on weakly-ordered archs

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Message-ID: <1521564718.13995.1.camel@redhat.com>
Date:   Tue, 20 Mar 2018 12:51:58 -0400
From:   Laurence Oberman <loberman@...hat.com>
To:     Sinan Kaya <okaya@...eaurora.org>, linux-scsi@...r.kernel.org,
        timur@...eaurora.org, sulrich@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Don Brace <don.brace@...rosemi.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        esc.storagedev@...rosemi.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/7] scsi: hpsa: Eliminate duplicate barriers on
 weakly-ordered archs

On Mon, 2018-03-19 at 22:50 -0400, Sinan Kaya wrote:
> Code includes wmb() followed by writel(). writel() already has a
> barrier on some architectures like arm64.
> 
> This ends up CPU observing two barriers back to back before executing
> the register write.
> 
> Since code already has an explicit barrier call, changing writel() to
> writel_relaxed().
> 
> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
> ---
>  drivers/scsi/hpsa.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/hpsa.h b/drivers/scsi/hpsa.h
> index 018f980..c7d7e6a 100644
> --- a/drivers/scsi/hpsa.h
> +++ b/drivers/scsi/hpsa.h
> @@ -599,7 +599,7 @@ static unsigned long
> SA5_ioaccel_mode1_completed(struct ctlr_info *h, u8 q)
>  		 * but with current driver design this is easiest.
>  		 */
>  		wmb();
> -		writel((q << 24) | rq->current_entry, h->vaddr +
> +		writel_relaxed((q << 24) | rq->current_entry, h-
> >vaddr +
>  				IOACCEL_MODE1_CONSUMER_INDEX);
>  		atomic_dec(&h->commands_outstanding);
>  	}

This looks like it would work for the x86_64 and arm because of how its
defined architecture specific for the x86_64 and the arm64

I guess its up to Don and the driver folks and if its worth the change.
I am generally not a fan of messing with these barrier things though.

Reviewed-by: Laurence Oberman <loberman@...hat.com>