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Quectel: Modem is released and forced to reconnect after PIN entry

Hi, There's an issue at least in ModemManager 1.20.0, which shows the following symptoms on a Pinephone & Pro, but I'm pretty sure it will affect any other Qualcomm modem:

With the modem booted and asking for a SIM PIN, after unlocking it ModemManager will release all the ports and disconnect all QMI sessions, then reconnect again. This makes the modem disappear for about 5 seconds from the UI. If you enter the pin with mmcli, mmcli will hang retrying to verify the SIM lock status until it times out.

The problem lays with the IMSI. As the PIN is sim-locked, the IMSI is not available at the start. When the SIM is unlocked the IMSI can be read, the sim hot-swap code in ModemManager thinks the card has been switched and it disconnects from the modem to reread everything:

dic 11 06:26:12 pipro ModemManager[1031]: <info>  [1670736372.390103] [modem19] SIM imsi has changed: <none> -> XXXXXXXXXXXXXX
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.390195] [modem19] releasing serial ports context for SIM hot swap
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.390269] [ttyUSB2/at] device open count is 0 (close)
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.390344] [ttyUSB2/at] closing serial port...
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.405920] [ttyUSB2/at] serial port closed
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.406160] [modem19] (quectel) check SIM swap completed
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.406906] [device /sys/devices/platform/fe3c0000.usb/usb2/2-1] unexported modem from path '/org/freedesktop/ModemManager1/Modem/19'
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.407080] [cdc-wdm0/qmi] Releasing client for service 'sar'...
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.407177] [/dev/cdc-wdm0] releasing 'sar' client with flags 'release-cid'...
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.407258] [/dev/cdc-wdm0] unregistered 'sar' client with ID '1'
dic 11 06:26:12 pipro ModemManager[1031]: <debug> [1670736372.407405] [/dev/cdc-wdm0] sent message...

I don't know ModemManager's code well enough to make a proper fix, but here's an example patch:

diff --git a/src/mm-broadband-modem.c b/src/mm-broadband-modem.c
index a9d4cd73..fa3d3709 100644
--- a/src/mm-broadband-modem.c
+++ b/src/mm-broadband-modem.c
@@ -4195,6 +4195,10 @@ complete_sim_swap_check (GTask       *task,
     } else if (ctx->step == SIM_SWAP_CHECK_STEP_IMSI_CHANGED) {
         ctx->imsi_check_done = TRUE;
         cached = mm_gdbus_sim_get_imsi (MM_GDBUS_SIM (ctx->sim));
+       if (!cached) {
+         mm_gdbus_sim_set_imsi (MM_GDBUS_SIM (ctx->sim), current);
+         cached = mm_gdbus_sim_get_imsi (MM_GDBUS_SIM (ctx->sim));
+       }
         str = "imsi";
     } else
         g_assert_not_reached();

I understand the IMSI going from NULL to a string should never trigger a hot-swap event, so this does just that. This example prevent mm_broadband_modem_sim_hot_swap_ports_context_reset from being called when the IMSI was null, so everything works as before hot-swap was used (Seems like this commit is where it was enabled)

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