netdev - Re: Socket Buffers and Memory Managment

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Date:	Wed, 18 Jul 2007 23:51:03 -0700 (PDT)
From:	vinay ravuri <vinaynyc@...oo.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
Cc:	netdev@...r.kernel.org
Subject: Re: Socket Buffers and Memory Managment

How about the following approach:

I allocate an skb of 0 bytes and replace data element
of skb struct (i.e. skb.data = addr_given_by_hw) when
the h/w interrupts me with a packet.  I register for a
destructor for this skb and when the kernel is ready
to free the skb, I make sure that my free is invoked -
Ofcourse this is assuming that their is a facility in
linux socket buffers to be able to do destructors.  Is
this approach a viable, if so, are any gottcha's?

-Vinay


--- Stephen Hemminger
<shemminger@...ux-foundation.org> wrote:

> On Tue, 17 Jul 2007 10:20:58 -0700 (PDT)
> vinay ravuri <vinaynyc@...oo.com> wrote:
> 
> > Hi,
> > 
> > I am fairly new to linux socket buffers and have
> the
> > following questions!
> > 
> > I am working with a custom ethernet MAC that does
> not
> > allow me to specify a particular memory location
> for
> > the h/w to DMA the packet into (Rx side). 
> Instead, it
> > has a pool of fixed size buffers with some h/w
> > specific headers around each buffer that are
> managed
> > by h/w and will pick a free buffer and DMA the
> packet.
> 
> Sounds like sucky hardware...
> You need to copy to a newly allocated skb, see
> 8139too.c
> 
> >  It appears dev_alloc_skb() actually allocates the
> > physical memory and doesn't allow the user to
> specify
> > the skb.data to something specific to what I want
> > which is a problem for me.  First is my assumption
> > correct that I am cannot pick an arbitrary
> skb.data
> > location in struct sk_buff?  I want to avoid
> copying
> > the dma'ed data into a new socket buffer as it is
> > expense.  Is there any ways around this problem?
> 
> You could play tricks with skb frags but it would be
> fragile
> and not worth the trouble. The problem is that the
> receive
> skb can stay in the system for a really long time
> (until the application
> reads the data) so your fixed size buffer pool in
> hardware
> would get exhausted.
>  
> > Also, if the h/w gives me a single packet in
> multiple
> > locations (i.e. non-contiguous chunks of memory),
> can
> > socket buffers handle chains of buffers?  I am
> looking
> > for a facility like mbuf's in netbsd where one can
> > chain multiple buffers together to make construct
> a
> > single packet.
> 
> Yes, skb frag list could be used for that but you
> don't
> want to do that. See above. Copy the data into an
> new
> skb and reserve any necessary bytes so IP header is
> aligned.  I.e. if using ethernet header (14 bytes),
> do
> skb_reserve(skb, 2) before copying the data.
> 



       
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