drivers: modem: bg9x: Has no means to update size of received packet.

[ycsin]

Describe the bug
This issue is to document an unresolved bug that I encountered when I first tried the ZephyrRTOS with the bg9x modem driver on my EC21 modm.

The current quectel-bg9x driver's recv isn't working as expected, as the recv's unsolicited cmd handler doesn't update the packet size for the socket, compared to the ublox-sara-r4 driver that does as a reference.

This is related to #34273, where the MQTT CONNACK acknowledgement from the broker can never be read by the offloaded socket's implementation, since the data to be read is always zero.

In earlier version of Quectel modems or its firmware, when the modem receives something, the modem reports the <connectID> without specifying the size. For the current quectel-bg9x driver to work on these modem, it is required to query the size of the received buffer using a separate AT cmd, likely in a work.

Some recent firmware (> 2020) for some of the modems have been updated to report the length together with the <connectID> if configured to do so. For these lucky modems with the correct firmware, it is possible to update the buffer size directly in the unsol recv parser just like the ublox-sara's one.

To Test

1. Build mqtt samples on a board using the bg9x driver
2. See error

Impact
bg9x driver will not be able to receive anything.

Additional context
I've tried to fix the issue last time, IIRC, MQTT QoS 0 works after the patch but QoS 1 likely fails and I forgot why. I have since move to use gsm_ppp modem.

The commit can be found here for whoever interested to pick up this, but it is very messy as it contains some CCLK and GNSS stuff. I came up with this with the help of @elliot-wdtl

Some important things:

1. The old on_cmd_unsol_recv is modified to submit a work to perform modem_qird_query_work
2. modem_qird_query_work will do the AT cmd to query the size of the received buffer and update the socket accordingly.
3. This patch was done when I didn't really comprehend how things work.

Quectel TCP/IP application notes:

• Old versions
• New version that can report length directly - Requires login

[elliot-wdtl]

Some recent firmware (> 2020) for some of the modems have been updated to report the length together with the if configured to do so.

This is interesting info. I'm working with bg95 and I have made quite a few changes which I haven't got to trying to upstream yet. I take advantage of configuring the modem to indicate the length so maybe this will be a problem if it's not available on lots of modems. I have also found that even if I configure it to indicate the length, it will not indicate the length on SSL sockets which is a sticky point for me. You also can't query the size of the receive buffer on SSL sockets so I have a different workaround.

[ycsin]

I have made quite a few changes which I haven't got to trying to upstream yet. I take advantage of configuring the modem to indicate the length so maybe this will be a problem if it's not available on lots of modems.

I guess for older firmware's users they will have to implement the AT+QIRD way, at least your current implementation will cover lots of newer firmware and modems, that is still a huge improvement to its current state, which doesn't work at all.

[github-actions]

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[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.