bce0: Error mapping mbuf into TX chain!

[Pyun YongHyeon]

On Wed, Aug 09, 2006 at 11:02:37AM +0200, Scott Wilson wrote:
 > On 8/8/06, David (Controller AE) Christensen <davidch at broadcom.com> wrote:
 > >>
 > >> Since BCE_MAX_SEGMENTS is too small I guess it will happen on highly
 > >> fragmented packets under heavy loads. To simulate the situation
 > >> you can use m_fragment(9) to fragment the frame in bce_tx_encap().
 > >> With m_fragment(9), "ping -f -s 65507 x.x.x.x" may trigger it.
 > >>
 > >
 > >I didn't know about m_fragment before.  I'll write a note to myself
 > >and look at how to add it to the debug path for a future driver
 > >revision.
 > >
 > >> Btw, I've never seen this small number of Tx DMA segments support(
 > >> BCE_MAX_SEGMENTS == 8) on GigE. Is this hardware limitation?
 > >>
 > >
 > >The real value for BCE_MAX_SEGMENTS should be 16, not 8.  I chose 8 as a
 > >reasonable value to start with.  If the number of fragments exceeds 16
 > >then we would expect to see performance drop and it is probably faster
 > >to
 > >have the OS defragment the packet rather than try to perform so many
 > >DMAs.
 > >
 > 
 > What I don't understand is why the driver stays locked up after it
 > gets into this mode.  I guess that's a separate issue from the low max
 > segments which is triggering it in the first place?
 > 

There are several cases here.
1. Due to lack of free Tx descriptors bus_dmamap_load_mbuf(9) can fail,
   so loaded DMA map should be unloaded with bus_dmamap_unload(9)
   in order to reload it after m_defrag(9) call.
2. If m_defrag(9) fail you may want to free m_head as keeping it
   in queue may result in stuck condition. Since m_defrag(9) can't
   defragment the mbuf chain you can never send it again if you
   requeue the mbuf chain.
3. If the second bus_dmamap_load_mbuf(9) fail you should requeue
   m_head which was alreay modified with m_defrag(9). Just returning
   error from failure make bce(4) reuse invalud mbuf chain.
As a general rule caller of m_defrag(9) should be prepared to cope
with modified mbuf chains when it requeues the mbuf chains.

-- 
Regards,
Pyun YongHyeon