netdev - RE: [PATCH 1/1] qlcnic: add wmb() call in transmit data path.

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Date:	Thu, 30 Jun 2016 18:48:25 +0000
From:	Sony Chacko <sony.chacko@...gic.com>
To:	Lino Sanfilippo <lsanfil@...vell.com>,
	David Miller <davem@...emloft.net>
CC:	netdev <netdev@...r.kernel.org>,
	Dept-NX Linux NIC Driver 
	<Dept_NX_Linux_NIC_Driver@...gic.com>
Subject: RE: [PATCH 1/1] qlcnic: add wmb() call in transmit data path.

Subject: Re: [PATCH 1/1] qlcnic: add wmb() call in transmit data path.

>
>> +    /* Ensure writes are complete before HW fetches Tx descriptors */
>> +    wmb();
>>       qlcnic_update_cmd_producer(tx_ring);
>>
>>       return NETDEV_TX_OK;
>>
>
> Would not an mmiowb be more appropriate in this case?
>
> Regards,
> Lino

Sorry, this was nonsense.This should be "dma_wmb" not "mmiowb".

Lino,

The patch is based on this kernel documentation. 

https://www.kernel.org/doc/Documentation/memory-barriers.txt
	
	/* force memory to sync before notifying device via MMIO */
		wmb();

		/* notify device of new descriptors */
		writel(DESC_NOTIFY, doorbell);
	}
The wmb() is needed to guarantee that the cache coherent memory writes have 
completed before attempting a write to the cache incoherent MMIO region.

Thanks,
Sony