linux-kernel - Re: [PATCH] af_packet: Don't use skb after dev_queue

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Message-ID: <4B587D4D.2010300@s5r6.in-berlin.de>
Date:	Thu, 21 Jan 2010 17:14:05 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
CC:	Michael Breuer <mbreuer@...jas.com>,
	Jarek Poplawski <jarkao2@...il.com>,
	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	flyboy@...il.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: Don't use skb after dev_queue_xmit()

Stephen Hemminger wrote:
> On Tue, 19 Jan 2010 20:01:10 -0500
> Michael Breuer <mbreuer@...jas.com> wrote:
>> On 1/19/2010 5:45 PM, Jarek Poplawski wrote:
[...]
>> > --- a/drivers/net/sky2.c
>> > +++ b/drivers/net/sky2.c
>> > @@ -2252,12 +2252,14 @@ static struct sk_buff *receive_copy(struct sky2_port *sky2,
>> >   	skb = netdev_alloc_skb_ip_align(sky2->netdev, length);
>> >   	if (likely(skb)) {
>> >   		pci_dma_sync_single_for_cpu(sky2->hw->pdev, re->data_addr,
>> > -					    length, PCI_DMA_FROMDEVICE);
>> > +					    pci_unmap_len(re, data_size),
>> > +					    PCI_DMA_FROMDEVICE);
>> >   		skb_copy_from_linear_data(re->skb, skb->data, length);
>> >   		skb->ip_summed = re->skb->ip_summed;
>> >   		skb->csum = re->skb->csum;
>> >   		pci_dma_sync_single_for_device(sky2->hw->pdev, re->data_addr,
>> > -					       length, PCI_DMA_FROMDEVICE);
>> > +					       pci_unmap_len(re, data_size),
>> > +					       PCI_DMA_FROMDEVICE);
>> >   		re->skb->ip_summed = CHECKSUM_NONE;
>> >   		skb_put(skb, length);
>> >   	}
>> >    
>> That not only compiled, but it cleared the error as well. Additionally, 
>> I used to see a bit of a delay receiving the login prompt when first 
>> connecting to the box by ssh. That delay is gone with this patch. I'd 
>> guess that the warning wasn't quite as innocuous as I thought.
>> Note: tested on 2.6.32.4.  I'll leave this up for a bit before 
>> attempting to move back to head.
> 
> 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, 

I didn't read the start of this thread and may misunderstand the issue,
but I comment anyway...:

Documentation/DMA-API.txt says the following about (pci_)dma_sync_single
and friends:  "All the parameters must be the same as those passed into
the single mapping API."
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/DMA-API.txt;h=5aceb88b3f8b622c8c23ed288f37f42de705c904;hb=HEAD#l367

So, it might be considered an API bug --- but it is a documented one. :-)
-- 
Stefan Richter
-=====-==-=- ---= =-=-=
http://arcgraph.de/sr/
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