cvs commit: src/sys/dev/em if_em.c

[Pyun YongHyeon]

On Sun, Jul 30, 2006 at 01:50:55PM +0400, Yar Tikhiy wrote:
 > On Sun, Jul 30, 2006 at 03:48:07PM +0900, Pyun YongHyeon wrote:
 > > On Sat, Jul 29, 2006 at 08:50:09PM +0000, Yar Tikhiy wrote:
 > >  > On Thu, Jul 27, 2006 at 12:43:34AM +0000, Pyun YongHyeon wrote:
 > >  > > yongari     2006-07-27 00:43:34 UTC
 > >  > > 
 > >  > >   FreeBSD src repository
 > >  > > 
 > >  > >   Modified files:
 > >  > >     sys/dev/em           if_em.c 
 > >  > >   Log:
 > >  > >   Prepending an mbuf after loading a DMA map results in unexpected
 > >  > >   result. So, modify mbuf chains before loading a DMA map.
 > >  > >   
 > >  > >   Revision  Changes    Path
 > >  > >   1.122     +28 -31    src/sys/dev/em/if_em.c
 > >  > 
 > >  > Thanks a lot!  Do you think this can fix kern/72933?
 > >  > 
 > > 
 > > I can't sure it helps as the submitter reported the same issue
 > > on bge(4).
 > 
 > Sorry, my question wasn't accurate.  I believe that the original
 > PR in fact described a different issue with similar symptoms; it
 > was fixed quite a while ago.  Then Lev Shamardin posted a follow-up
 > that he was still seeing the symptoms with em(4), which we (Gleb
 > Smirnoff and yours truly) couldn't fully understand because we were
 > missing the peculiarities of the interaction between DMA and mbufs
 > you're so well aware of.
 > 
 > > Btw, I think the VLAN fixup code should use m_prepend(9)
 > > insatead of M_PREEND(9) because we don't know whether a new mbuf
 > > is allocated or not after M_PREPEND(9) call.
 > 
 > If a new mbuf should be allocated to satisfy DMA, m_prepend(9) is

No, prepending a new mbuf with m_prepend(9) is not necessary for DMA
to work.

 > the function to use.  M_PREPEND(9) can use the free space at the
 > beginning of the mbuf's data area if there is enough of it.  I'm
 > unsure though whether we really need a new mbuf there.  em_encap()
 > gets just a mbuf chain, which can be tweaked a little before starting
 > the DMA magic on it.
 > 

Sorry, I'm confused. I've misread VLAN fixup code. From my limited testing, 
it seems that it works as expected.
Previously em(4) may have failed VLAN tag insertion if M_PREPEND(9) added
a new mbuf into existing mbuf chains. 

 > -- 
 > Yar

-- 
Regards,
Pyun YongHyeon