commit 776c5b16ea63eb80349379fffb39e52ee6b2024d
author Vadim Yanitskiy <vyanitskiy@sysmocom.de> Thu Aug 27 20:58:49 2020 +0700
committer Vadim Yanitskiy <vyanitskiy@sysmocom.de> Thu Aug 27 22:10:54 2020 +0700
tree 8cc1b9ab38c395ad1755cdac210644f6911b181c
parent 488989d2233e8e5a9f1cf33466da2f5db7e5b0f4

lapdm: fix SAPI-0/SAPI-3 frame prioritization on DCCH

According to 3GPP TS 44.005, section 4.2.2 "Priority":

  a) on DCCH, a SAPI=0 frame always has higher priority than SAPI=3;
  b) on ACCH, the priority arrangement is more complex:
    b1) if a SAPI = 3 frame is awaiting transmission, two SAPI=0
        frames shall not be sent in consecutive SACCH frames;
    b2) on the network side (LAPDM_MODE_BTS), it must also be ensured
        that any SAPI=3 frame is followed by at least one SAPI=0 frame;
    b3) a SAPI = 0 frame may be repeated in the next SACCH period
        if the Repeated SACCH is supported (see 3GPP TS 44.006, section 11).

We definitely need to extend our testing coverage to ensure that
we implement b) correctly, but for now let's focus on DCCH:

  a) for DCCH, ensure that SAPI=0 frames preceed SAPI=3 ones;
  b) for ACCH, re-use the existing round-robin implementation.

Change-Id: Ia3780bce1222b312ae2fd2d21496a4d6c5ccb6e0
Related: SYS#5047, OS#4731