netdev - RE: [PATCH net-next v3 2/2] enic: use netdev_dma

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Date:	Wed, 11 Mar 2015 17:34:23 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alexander Duyck' <alexander.duyck@...il.com>,
	Govindarajulu Varadarajan <_govind@....com>,
	Alexander Duyck <alexander.h.duyck@...hat.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"ssujith@...co.com" <ssujith@...co.com>,
	"benve@...co.com" <benve@...co.com>
Subject: RE: [PATCH net-next v3 2/2] enic: use netdev_dma_alloc

From: Alexander Duyck
...
> > Is this behaviour platform dependent? I tested this patch for more
> > than a month
> > and I did not face any issue. I ran normal traffic like ssh, nfs and
> > iperf/netperf.
> > Is there a special scenario when this could occur?
> 
> Yes it depends on the platform and IOMMU used.  For an example take a
> loot at the SWIOTLB implementation.  I always assumed if I can work with
> that when it is doing bounce buffers I can work with any IOMMU or platform.
> 
> >
> > Will using DMA_BIDIRECTIONAL and sync_to_cpu & sync_to_device solve this?
> > Each desc should have different dma address to write to. Can you
> > explain me how
> > this can happen?
> 
> No that won't help.  The issue is that when the page is mapped you
> should not be updating any fields in the page until it is unmapped.
> Since you have multiple buffers mapped to a single page you should be
> waiting until the entire page is unmapped.

Isn't the 'unit of memory for dma sync' a cache line, not a page?

You certainly need to test on systems without cache coherent io.

	David

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