Date:	Wed, 27 Jan 2010 12:58:59 -0500
From:	Michael Breuer <mbreuer@...jas.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
Cc:	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, Michael Chan <mchan@...adcom.com>,
	Don Fry <pcnet32@...izon.net>,
	Francois Romieu <romieu@...zoreil.com>,
	Matt Carlson <mcarlson@...adcom.com>
Subject: Re: Hang: 2.6.32.4 sky2/DMAR (was [PATCH] sky2: Fix WARNING: at
 lib/dma-debug.c:902 check_sync)

On 1/27/2010 12:56 PM, Stephen Hemminger wrote:
> On Wed, 27 Jan 2010 11:57:35 -0500
> Michael Breuer<mbreuer@...jas.com>  wrote:
>
>    
>> On 1/27/2010 11:50 AM, Stephen Hemminger wrote:
>>      
>>> On Wed, 27 Jan 2010 10:34:51 -0500
>>> Michael Breuer<mbreuer@...jas.com>   wrote:
>>>
>>>
>>>        
>>>> On 01/23/2010 06:21 PM, Jarek Poplawski wrote:
>>>>
>>>>          
>>>>> On Fri, Jan 22, 2010 at 06:50:21PM -0500, Michael Breuer wrote:
>>>>>
>>>>>
>>>>>            
>>>>>> When the packets were dropped, there was a different sequence in the
>>>>>> log - DISCOVER/OFFER repeated. The "normal" is that the sequence
>>>>>> appeared correct and complete - DISCOVER/OFFER/REQUEST/ACK - or
>>>>>> INFORM/ACK (vs. INFORM repeatedly sans ACK) as the case may be.
>>>>>>
>>>>>>
>>>>>>              
>>>>> Anyway, I'd be intersted if the switch matters here.
>>>>>
>>>>> Plus one more test: could you try to load sky2 with the parameter:
>>>>> "copybreak=1" (the rest as in any recent test, which gave you dmar
>>>>> errors; any switch).
>>>>>
>>>>> Thanks,
>>>>> Jarek P.
>>>>>
>>>>>
>>>>>            
>>>> Ok - now up 80+ hours with copybreak=1. I'm going to redo w/o copybreak
>>>> to confirm that I haven't inadvertently fixed something. However, given
>>>> that it might be copybreak-related, I looked at sky2.c again and I'm
>>>> wondering about the copybreak max size in sky2_rx_start:
>>>>
>>>>      size = roundup(sky2->netdev->mtu + ETH_HLEN + VLAN_HLEN, 8);
>>>>
>>>>            /* Stopping point for hardware truncation */
>>>>            thresh = (size - 8) / sizeof(u32);
>>>>
>>>>            sky2->rx_nfrags = size>>   PAGE_SHIFT;
>>>>            BUG_ON(sky2->rx_nfrags>   ARRAY_SIZE(re->frag_addr));
>>>>
>>>>            /* Compute residue after pages */
>>>>            size -= sky2->rx_nfrags<<   PAGE_SHIFT;
>>>>
>>>>            /* Optimize to handle small packets and headers */
>>>>            if (size<   copybreak)
>>>>                    size = copybreak;
>>>>            if (size<   ETH_HLEN)
>>>>                    size = ETH_HLEN;
>>>>
>>>>
>>>> Why would increasing size to copybreak be valid here?
>>>>
>>>> Guessing a bit as I'm not sure about rx_nfrags, but if I read this
>>>> correctly, if size is ever less than copybreak it's because there isn't
>>>> enough space left for anything larger. If so, wouldn't increasing size
>>>> potentially corrupt something? I'd further guess that the resulting
>>>> condition manifests sooner (or at least with a more visible effect) when
>>>> using DMAR.
>>>>
>>>> In any event, why "copybreak" as the minimum buffer size? I'd suggest
>>>> that if it isn't possible to allocate at least MTU + overhead that
>>>> sky2_rx_start ought to be delayed until there is room.
>>>>
>>>>          
>>> This code is where driver decides how much data will be received in skb
>>> data area and the remaining data spills over into skb frags.
>>> Copybreak is the threshold so that packets less than size are copied
>>> to a new skb.  The code doing the copying there assumes the data is
>>> totally contained in the skb (not in frags). The size increase there
>>> is to make sure that assumption is always true.  I suppose you
>>> could do something perverse like setting copybreak really huge
>>> and confuse driver, but that is a user error.
>>>
>>>
>>>        
>> Ok - but I'm wondering under what circumstances size would be<
>> copybreak in the first place after computing the residue. If size ends
>> up being unreasonably small, is simply increasing the number to whatever
>> copybreak is correct? Assuming my testing is correct, then the crash
>> I've been experiencing when using dmar (only) seems related to the value
>> of copybreak. I don't think the other use (skb reuse) is the issue (but
>> hey, I could have missed something). The crash occurs when copybreak is
>> the default of 128, didn't happen when I set copybreak to 1.
>>      
> Does this change it? If so the dma code is (not sky2) is buggy and not
> rounding up properly.
>
> --- a/drivers/net/sky2.c	2010-01-27 09:46:10.940005248 -0800
> +++ b/drivers/net/sky2.c	2010-01-27 09:53:47.141267850 -0800
> @@ -2257,13 +2257,16 @@ static struct sk_buff *receive_copy(stru
>
>   	skb = netdev_alloc_skb_ip_align(sky2->netdev, length);
>   	if (likely(skb)) {
> +		unsigned dma_align = dma_get_cache_alignment();
> +		unsigned dma_size = ALIGN(length+1, dma_align);
> +
>   		pci_dma_sync_single_for_cpu(sky2->hw->pdev, re->data_addr,
> -					    length, PCI_DMA_FROMDEVICE);
> +					    dma_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);
> +					       dma_size, PCI_DMA_FROMDEVICE);
>   		re->skb->ip_summed = CHECKSUM_NONE;
>   		skb_put(skb, length);
>   	}
>    
Ok - will queue this - want to reconfirm that the system still crashes 
w/o this (or copybreak). That should take a few days.
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