netdev - Re: [Bugme-new] [Bug 38102] New: BUG kmalloc-2048: Poison overwritten

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Date:	Tue, 5 Jul 2011 15:53:53 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexey Zaytsev <alexey.zaytsev@...il.com>,
	Michael Büsch <m@...s.ch>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, Gary Zambrano <zambrano@...adcom.com>,
	bugme-daemon@...zilla.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Pekka Pietikainen <pp@...oulu.fi>,
	Florian Schirmer <jolt@...box.org>,
	Felix Fietkau <nbd@...nwrt.org>, Michael Buesch <mb@...sch.de>
Subject: Re: [Bugme-new] [Bug 38102] New: BUG kmalloc-2048: Poison overwritten

On Tue, Jul 05, 2011 at 08:45:16PM +0200, Eric Dumazet wrote:
> Le mardi 05 juillet 2011 à 20:32 +0200, Eric Dumazet a écrit :
> 
> > Then, maybe the driver model is completely wrong, and should really
> > setup 512 buffers, or use less descs but set EOT on last one.
> > 
> > Currently it uses a 200 sliding window out of the 512 descs.
> > 
> > 
> 
> One thing we could do would be to allocate a special guard buffer and
> set all 'out of window' descriptors to point to this guard buffer, and
> periodically check if buffer is dirtied by the card.
> 
> (first word would be enough)
> 
> (instead of setting desc->addr to NULL, set to
> dma_map_single(guard_buffer))
> 
I think this is a goo idea, at least for testing.  It seems odd to me that we
have the B44_DMARX_PTR value which indicates (ostensibly) the pointer to the
descriptor to be processed next (the documentation isnt' very verbose on the
subject), along with the EOT bit on a descriptor.  It seems like both the
register and the bit are capable of conveying the same (or at least overlapping)
information.

I think what I'm having the most trouble with is understanding when the hw looks
at the EOT bit in the descriptor.  If it completes a DMA and sees the EOT bit
set, does the next DMA occur to the descriptor pointed to by the DMARX_ADDR
register?  Of does it stall until such time as the DMARX_PTR register is rotated
around?  What if it doesn't see the EOT bit set?  Does it just keep going with
the next descriptor?  

Also, there seems to be some inconsistency in the settnig of the B44_DMARX_PTR
register.  In bnx2_init_hw its set to the value of bp->rx_pending, which is
defined as being 200.  But in b44_rx its advanced by sizeof(struct dma_desc) for
every iteration.  So in b44_init_hw we write the value 200 to it, ostensibly
indicating a limit of 200 descriptors, but in b44_rx we iteratively write the
values 0, 8, 16, 24...4*n to the register to indicate which descriptor we're
indexing?  Something really doesn't sit right with me there.  In the former case
we treat the register as holding  number of entries, and in the latter we treat
it as holding a byte offset into an array.  Or am I missing something?

Regards
Neil

> 
> 
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