[ixp1200] Fast Ports - sop_seq roll over

[Yannis Haritakis]

Hi,

 Based on the example of Fast Port usage of the book
programming IXP1200 (intel press), I made the following observation.
(If familiar please respond).



 The example uses sop_seq field from rcv_ctl register in order to put
the MPKTs of a PACKET in the correct order.

 However, in general, using the sop_seq allows for only 16 different
 MPKTS.
 That is, the number rolls over after 0xF.
 A 1500bytes PACKET results in 24 MPKTS.
 If more than one thread handle these MPKTS in parallel, there is
 a possibility for two threads to receive the same sop_seq.



 Another relative issue is the next one :

 In the book it is written that the sop_seq field of rcv_cntl register,
follows the values of the sop_seq register. This is wrong.

 The sop_seq register changes only when an SOP-MPKT is received, while
the sop_seq field in rcv_cntl register behaves as described in the book.
(Always increases, and get the sop_seq value when a SOP-MPKT is received).
To support this behavior there are two counters (documented in hardware
reference manual)

 Now the question is this :

 If a thread, after the ctx_wait(start_receive) executes two back-to-back
csr_reads :
	csr_read(sop_seq, sync_none);
	csr_read(rcv_cntl, ctx_wait);

will end up having two sop_seq: The value of the register
and the field of rcv_cntl.

 Will the value of the register be the sop-seq number that was assigned
to the PACKET that this thread is working for ?


 I think the answer is "no necessarily"!

Yannis Charitakis
haritak at ics.forth.gr