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

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Message-ID: <1264018312.9030.76.camel@nseg_linux_HP1.broadcom.com>
Date:	Wed, 20 Jan 2010 12:11:52 -0800
From:	"Michael Chan" <mchan@...adcom.com>
To:	"Stephen Hemminger" <shemminger@...ux-foundation.org>
cc:	"Jarek Poplawski" <jarkao2@...il.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, 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.

Thanks.

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