Change in osmo-pcu[master]: Implement downgrade to DL MCS1-4 when USF for GPRS_only MS

[pespin]

Hello Jenkins Builder, neels, laforge, fixeria, lynxis lazus, 

I'd like you to reexamine a change. Please visit

    https://gerrit.osmocom.org/c/osmo-pcu/+/21179

to look at the new patch set (#5).

Change subject: Implement downgrade to DL MCS1-4 when USF for GPRS_only MS
......................................................................

Implement downgrade to DL MCS1-4 when USF for GPRS_only MS

In previous status, if USF for GPRS-only MS was selected, then EGPRS
TBFs were skipped and either a GPRS TBF was selected or a Dummy Block
was sent. That means the behavior was unfair towards EGPRS TBFs, because
sometimes they were skipped in favor of GPRS ones.
This patch imporves the situation in the above mentioned USF scenario, by
first, under specific conditions, allowing selection of an EGPRS TBF and
then forcing it to transmit in EGPRS-GMSK (MCS1-4) so that the
USF-targeted MS can still decode the USF, while at the same time
providing more fairness by allowing the EGPRS TBF to transmit data.

The specific conditions mentioned above are, mainly, related to the fact
that once a DL data block has been sent, and hence a BSN was assigned to
it, it cannot be retransmitted later using another MCS, since lower
MCS1-4 wouldn't be able to contain higher MCS RLC payload.

The set of conditions could be expanded in the future by also selecting
the EGPRS TBF if retransmition is required and the block to be
retransmitted was originally transmitted as MCS1-4.

Related: OS#4544
Change-Id: I9af23e175435fe9ae7b0e4119ad52fcd4707b9ca
---
M src/bts.cpp
M src/bts.h
M src/gprs_rlcmac_sched.cpp
M src/tbf_dl.cpp
M src/tbf_dl.h
M tests/tbf/TbfTest.err
6 files changed, 463 insertions(+), 431 deletions(-)


  git pull ssh://gerrit.osmocom.org:29418/osmo-pcu refs/changes/79/21179/5
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Gerrit-Project: osmo-pcu
Gerrit-Branch: master
Gerrit-Change-Id: I9af23e175435fe9ae7b0e4119ad52fcd4707b9ca
Gerrit-Change-Number: 21179
Gerrit-PatchSet: 5
Gerrit-Owner: pespin <pespin at sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: fixeria <vyanitskiy at sysmocom.de>
Gerrit-Reviewer: laforge <laforge at osmocom.org>
Gerrit-Reviewer: lynxis lazus <lynxis at fe80.eu>
Gerrit-Reviewer: neels <nhofmeyr at sysmocom.de>
Gerrit-MessageType: newpatchset
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