fix ip.access dyn TS for osmo-bts-trx

For enabling PDCH on a dynamic timeslot, PDTCH and PTCCH SAPIs shall be
enabled. While osmo-bts-sysmo uses the lchan->type to determine which SAPIs to
enable (see lchan_activate() in osmo-bts-sysmo/oml.c:
sapis_for_lchan[lchan->type]), the osmo-bts-trx code instead relies on the
chan_nr indicating RSL_CHAN_OSMO_PDCH = 0xc0 (see trx_sched_set_lchan() in
common/scheduler.c and the PDTCH,PTCCH entries in trx_chan_desc[]).

The 0xc0 cbits are a non-standard invention specifically used for only Osmocom
style dyn TS, so the chan_nr for IPA style dyn TS will and should never include
this cbits pattern. Hence gsm_lchan2chan_nr() correctly always returns the
TCH/F equivalent chan_nr for IPA dyn TS.

Because trx_chan_desc[] relies on the 0xc0 in the chan_nr to activate the PDTCH
and PTCCH SAPIs, internally patch the 0xc0 cbits over the chan_nr in
osmo-bts-trx/l1_if.c for channel de-/activation, iff lchan->type == PDTCH.

This is technically a convoluted mix-up of the cbits usage. Nevertheless, it is
the simplest way to make IPA dyn TS behave the same as Osmocom dyn TS in
scheduler.c.

Apparently, IPA style dyn TS have never worked for osmo-bts-trx before?

Related: OS#3493
Change-Id: I0eed8a135f2ab7e7c0d15ad5c76430b7fe54df3d

0 files changed, 0 insertions, 0 deletions