netdev - Re: [PATCH] be2net: Bugfix for packet drop with kernel param swiotlb=force

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Message-ID: <1393347489.14067.5.camel@deadeye.wl.decadent.org.uk>
Date:	Tue, 25 Feb 2014 16:58:09 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Sathya Perla <Sathya.Perla@...lex.Com>
Cc:	"jiang.biao2@....com.cn" <jiang.biao2@....com.cn>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Subramanian Seetharaman <subbu.seetharaman@...lex.com>,
	Ajit Khaparde <Ajit.Khaparde@...lex.Com>,
	"wang.liang82@....com.cn" <wang.liang82@....com.cn>,
	"cai.qu@....com.cn" <cai.qu@....com.cn>,
	"li.fengmao@....com.cn" <li.fengmao@....com.cn>,
	"long.chun@....com.cn" <long.chun@....com.cn>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] be2net: Bugfix for packet drop with kernel param
 swiotlb=force

On Tue, 2014-02-25 at 14:06 +0000, Sathya Perla wrote:
> > -----Original Message-----
> > From: Ben Hutchings [mailto:ben@...adent.org.uk]
> > 
> > On Thu, 2014-02-20 at 09:39 +0000, Sathya Perla wrote:
> > > > -----Original Message-----
> > > > From: jiang.biao2@....com.cn [mailto:jiang.biao2@....com.cn]
> > > >
> > > >
> > > > From: Li Fengmao <li.fengmao@....com.cn>
> > > >
> > > > There will be packet drop with kernel param "swiotlb = force" on
> > > > Emulex 10Gb NIC using be2net driver. The problem is caused by
> > > > receiving skb without calling pci_unmap_page() in get_rx_page_info().
> > > > rx_page_info->last_page_user is initialized to false in
> > > > be_post_rx_frags() when current frag are mapped in the first half of
> > > > the same page with another frag. But in that case with
> > > > "swiotlb = force" param, data can not be copied into the page of
> > > > rx_page_info without calling pci_unmap_page, so the data frag mapped
> > > > in the first half of the page will be dropped.
> > > >
> > > > It can be solved by creating only a mapping relation between frag
> > > > and page, and deleting rx_page_info->last_page_user to ensure
> > > > calling pci_unmap_page when handling each receiving frag.
> > >
> > > This patch uses an entire page for each RX frag (whose default size is 2048).
> > > Consequently, on platforms like ppc64 where the default PAGE_SIZE is 64K,
> > > memory usage becomes very inefficient.
> > >
> > > Instead, I've tried a partial-page mapping scheme. This retains the
> > > page sharing logic, but un-maps each frag separately so that
> > > the data is copied from the bounce buffers.
> > [...]
> > 
> > You don't need to map/unmap each fragment separately; you can sync a
> > sub-page range with dma_sync_single_for_cpu().
> > 
> 
> Ben, after syncing each frag with a dma_sync_single_for_cpu() call,
> would I still need to dma_unmap_page() after DMA is done on all the frags
> of the page? I thought I needed to.

Yes.

> I'm confused to see that swiotlb_bounce() (that copies data from the bounce buffers)
> is called from both swiotlb_sync_single_for_cpu() and swiotlb_unmap_page().
> I ofcourse don't want the data to be copied from the bounce buffers twice!

Right, unmap includes a sync so you should:
- for the last frag per page, dma_unmap_page() only
- for every other frag, dma_sync_single_for_cpu() only

Ben.

-- 
Ben Hutchings
Everything should be made as simple as possible, but not simpler.
                                                           - Albert Einstein

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