- to fetch PDU session to setup in E1 following logic as in e1_send_bearer_updates
- fill bearer context modification request with stored F1-U tunnel information
  during E1 reestablishment, otherwise it would initialized to 0 by default
- update GTP tunnel with the stored configuration after E1 reestablishment

- this function refers to the E1 Bearer Context Modification Request
- therefore it has to return pdu_session_to_mod_t*
- and takes in input e1ap_bearer_mod_req_t *

- info coming from UE context setup/modification response messages in RRC

The previous code removed the "secondary UE" data (about DU association,
DU UE ID, associated CU UP), and only filled the F1-related data. In
other words, the associated CU-UP was lost. This commit reworks this
piece of code to retrieve old data, and update the relevant data without
destroying other.

- PDCP reestablishment on CUCP (DRBs) triggering Bearer Context Modification procedures over E1
- Performing PDCP reestablishment for requested DRBs on CUUP
- Introduced function to notify re-establishment to CU-UP
- removed call to PDCP reestablishment for the DRBs on CUCP (it's done on CUUP)

- many structs and definitions are overlapping between the two different E1 procedures
- introduced naming specific to E1 Bearer Context Modification to improve readability
- grouped SDAP and PDCP configuration IEs for better reusability and readability
- introduced functions to set and get default PDCP config (DRBs and Bearer Contexts)

see !MR2545 for more context

- new folder dedicated to E1AP docs
- new file to document E1AP procedures

- according to specs, in NR there are 4 SRBs
- in OAI the relevant define is used in the RRC procedures
- this commit harmonizes code by removing redundancy

- the goal is to improve maintanability

Integration: 2024.w06

Closes #733 and #732

See merge request oai/openairinterface5g!2564

* !2490 reworking configuration of LogicalChannelConfig at MAC UE
* !2548 NR UE MSG3 buffer
* !2556 Support RC SM aperiodic subscription for "UE RRC State Change"
* !2557 fix duplicate call of RCconfig_NR_L1
* !2550 use pointer to structure instead of module_id inside MAC
* address #733
* !2560 chore(ci): force rebuild of ran-base for integration branches
* !2495 Sidelink configuration passed from RRC->MAC
* !2220 L1 tx thread
* !2559 FH 7.2 improvements
* !2566 Provide right E1 instance when creating E2 agent in CU-UP
* !2513 Update NR SA tutorials

CUuniqInstance is also used for GTP module identification, which
uses the UDP server socket as its ID. This leads to arbitrary IDs (>0),
which is wrong.

Correctly show the path: it was missing cmake_targets/, although it is
there.

Closes: #733

Not periodically checking stats freeing can lead to asserts if these
lists are full. Check it once a frame to ensure that lists remain
non-empty.

	- Phy configuration will be prepared by MAC
	- Sidelink preconfiguration parameters passed from RRC->MAC
	- Only 1 SSB TA allocation used
	- psbch payload prepared by MAC after receiving the tx slss req

PUSCH decoding is started multiple times in nr_ulsch_procedures() for
each ULSCH segment, and stopped once inside
phy_procedures_gNB_uespec_RX(). Not only is it not clear if that works
with the API, it also might measure more than just PUSCH decoding.

Remove the PUSCH decoding, it is misleading. Check L1 RX processing
instead.

The CI would only detect a deviation from the normalized value based on
a configurable threshold, typically 1.25 (so we would tolerate a 25%
increase). However, it often happens that a timing gets shorter (e.g.,
forgot to put the timing measurements). This commit introduces a
"Deviation Threshold" (smaller than 1) to not only check that we exceed
1.0 + "Deviation Threshold" (as before), but also that are not below 1.0
- "Deviation Threshold".

In other words, assuming a maximum divergence of 25%, instead of just
checking if val > 1.25, we now also check if val < 0.75 (and flag
error).

This commit introduces a separate thread (l1_tx_thread) that processes
gNB DL/TX slots. It receives a message from the ru_thread when a new
slot started, and starts processing.

The DL part of the scheduler is run in the l1_tx_thread.  Therefore,
only call UL indication (for scheduler) in UL slots.  The UL indication
previously triggered the DL scheduler -- hence, it had to be called in
every slot. Now, since the DL scheduler is moved into the DL thread, we
don't have to call the scheduler in every slot anymore.

The reorder thread is removed, as reordering with this scheme is not
necessary anymore.

The main advantage of this version is that the TX can be scheduled
earlier (sl_ahead). Further, there can no longer be race conditions in
the TX path, since the scheduler/L2, TX L1 processing and the RU tx
function are all execute in the same thread (L1_tX_thread).

Also, the scheduler prepares PRACH/PUSCH/PUCCH FAPI PDUs. As of this
commit, the scheduler runs in the TX processing chain, whereas these
PDUs are destined for RX processing.  Thus, too avoid data races,
instead of triggering the RX processing at the same time as TX
processing in the RU thread, this commit changes the code to trigger RX
processing after the scheduler has been run in TX processing to avoid
data races.

Finally, we synchronize RU and L1 threads. This is important for
rfsimulator, since unlike most radios, rfsimulator can run slower or
faster, depending on I/O and CPU (it does not have a stable "tick").
Co-authored-by: kiran <saikiran@iitj.ac.in>

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>