netdev - Re: [PATCH] sky2: Fix WARNING: at lib/dma-debug.c:902 check

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Message-ID: <20100120205842.GA3072@del.dom.local>
Date:	Wed, 20 Jan 2010 21:58:42 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
Cc:	Michael Chan <mchan@...adcom.com>,
	David Miller <davem@...emloft.net>,
	Michael Breuer <mbreuer@...jas.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"flyboy@...il.com" <flyboy@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Don Fry <pcnet32@...izon.net>,
	Francois Romieu <romieu@...zoreil.com>,
	Matthew Carlson <mcarlson@...adcom.com>
Subject: Re: [PATCH] sky2: Fix WARNING: at lib/dma-debug.c:902 check_sync

On Wed, Jan 20, 2010 at 12:30:33PM -0800, Stephen Hemminger wrote:
> On Wed, 20 Jan 2010 12:11:52 -0800
> "Michael Chan" <mchan@...adcom.com> wrote:
> 
> > 
> > On Wed, 2010-01-20 at 10:03 -0800, Stephen Hemminger wrote:
> > > On Wed, 20 Jan 2010 09:41:03 +0000
> > > Jarek Poplawski <jarkao2@...il.com> wrote:
> > > > > Seems like an underlying bug in the DMA api. Maybe it just can't
> > > > > handle operations on partial mapping.
> > > > > 
> > > > > Other drivers with same problem:
> > > > >   bnx2, cassini, pcnet32, r8169, rrunner, skge, sungem, tg3, 
> > > > 
> > > > It seems using the same length (even without pci_unmap_len()) is
> > > > crucial here, but I hope maintainers (added to CC) will take care.
> > > > 
> > 
> > I'm still unsure how to do dma_sync properly in bnx2.  In the current
> > code, we always dma_sync_for_cpu a small portion of the SKB because the
> > rx descriptor is at the beginning of the SKB.  We get the packet length,
> > for example, from the rx descriptor.
> > 
> > If it's a big packet, we'll simply unmap the entire SKB buffer (with the
> > beginning portion already dma_sync'ed).  If the packet is smaller than
> > what we dma_sync'ed, we'll just copy the data to a new SKB.  We'll then
> > dma_sync_for_device the portion of the original buffer and recycle the
> > whole buffer back to the device for new packets.
> > 
> > So, is it correct to just change the dma_sync length to the full length
> > of the buffer?  It doesn't sound right to me.
> 
> It looks like the size passed to sync_single has to match size of original
> mapping.

Yes, and it's mainly for lib/dma-debug (until it's not verified dmar
errors reported by Michael Breuer could be connected). So, I'm not
sure for now how serious this warning could be. On the other hand,
Documentation/(PCI-?)DMA-mapping, mentioned by Stephen, doesn't seem
to allow or use in examples the "size" different than mapped.

Jarek P.
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