Fix #13490 Cellular disconnect does not deactivate context

[boraozgen]

Summary of changes

Fixes #13490, where the PDP context does not get deactivated in some cases when CellularInterface::disconnect() is called. Further description is in the issue.

Impact of changes

Migration actions required

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@boraozgen, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[AriParkkila]

The comment above For EPS, if an attempt is made to disconnect the last PDN connection, then the MT responds with ERROR... comes from 3GPP TS 27.007.

Unlike EPS, NB-IoT can be without PDN so could be fixed above like if (_is_context_active && (rat < CellularNetwork::RAT_E_UTRAN || rat == CellularNetwork::RAT_NB1 || active_contexts_count > 1)) {.

[boraozgen]

Added this. Does this also apply to CAT-M1?

[0xc0170]

Thanks @AriParkkila for review

[0xc0170]

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[ocecelja]

Added this. Does this also apply to CAT-M1?

Good question. According to changes it doesn't. But I'm pretty sure it should because CAT M1 and NB1 have similar working concepts from fw point of view, both needing contexts.

So maybe instead of:
if (_is_context_active && (rat < CellularNetwork::RAT_E_UTRAN || rat == CellularNetwork::RAT_NB1 || active_contexts_count > 1)) {
use:
if (_is_context_active && (rat < CellularNetwork::RAT_UNKNOWN || active_contexts_count > 1)) {

[boraozgen]

All RATs satisfy the condition rat < CellularNetwork::RAT_UNKNOWN. What do you mean by that?

[ocecelja]

As do for rat < CellularNetwork::RAT_E_UTRAN || rat == CellularNetwork::RAT_NB1 except CellularNetwork::RAT_CATM1. What I propose is to include RAT_CATM1.

Question is why did you left out RAT_CATM1?

[boraozgen]

I am not 100% sure but I think this issue was not affecting CAT-M1. I will try to test CAT-M1 with this fix and comment here.