linux-kernel - Re: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Fri, 23 Mar 2018 12:31:12 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, timur@...eaurora.org,
        sulrich@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, ariel.elior@...ium.com,
        everest-linux-l2@...ium.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on
 weakly-ordered archs

On 3/23/2018 12:20 PM, David Miller wrote:
> From: Sinan Kaya <okaya@...eaurora.org>
> Date: Thu, 22 Mar 2018 13:10:00 -0400
> 
>> Code includes wmb() followed by writel(). writel() already has a
>> barrier on some architectures like arm64.
>  ...
>> @@ -4155,7 +4155,7 @@ netdev_tx_t bnx2x_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>  	txdata->tx_db.data.prod += nbd;
>>  	barrier();
>>  
>> -	DOORBELL(bp, txdata->cid, txdata->tx_db.raw);
>> +	DOORBELL_RELAXED(bp, txdata->cid, txdata->tx_db.raw);
>>  
>>  	mmiowb();
>  ...
>> @@ -2592,7 +2592,7 @@ static int bnx2x_run_loopback(struct bnx2x *bp, int loopback_mode)
>>  
>>  	txdata->tx_db.data.prod += 2;
>>  	barrier();
>> -	DOORBELL(bp, txdata->cid, txdata->tx_db.raw);
>> +	DOORBELL_RELAXED(bp, txdata->cid, txdata->tx_db.raw);
> 
> These are compiler barriers being used here, not wmb().
> 
> Look, if I can't see a clear:
> 
> 	wmb()
> 	writel()
> 
> sequence in the patch hunks, I am going to keep pushing back on
> these changes.

Sorry, you got me confused now.

If you look at the code closer, you'll see this.

	wmb();

	txdata->tx_db.data.prod += nbd;
	barrier();

	DOORBELL(bp, txdata->cid, txdata->tx_db.raw);

and you also asked me to rename DOORBELL to DOORBELL_RELAXED() to make
it obvious that we have a relaxed operator inside the macro.

Did I miss something?

of course, treating barrier() universally as a write barrier is wrong.

> 
> Thank you.
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.