[LG V20 Guides, News, & Discussion] emdroidle:
I've been trying to avoid exposure to social networks to the greatest possible extent. Something about privacy...

Well, what I've written describes the situation. From the system log, the kernel attempts to start the modem, but that process terminates with an error:

M-Notify: General: 4
IPA received MPSS BEFORE_POWERUP
IPA BEFORE_POWERUP handling is complete
subsys-restart: subsystem_powerup(): [ffffffc0f7c38b00]: Powering up modem
modem is powering up
pil-q6v5-mss 2080000.qcom,mss: modem: loading from 0x0000000088000000 to 0x000000008e900000
M-Notify: General: 6
pil-q6v5-mss 2080000.qcom,mss: pil : mss reset and load mba
pil-q6v5-mss 2080000.qcom,mss: Debug policy not present - msadp. Continue.
pil-q6v5-mss 2080000.qcom,mss: Loading MBA and DP (if present) from 0x00000000f6f00000 to 0x00000000f7000000 size 100000
pil-q6v5-mss 2080000.qcom,mss: MBA boot done
pil-q6v5-mss 2080000.qcom,mss: pil : msa auth modem.mdt.
pil-q6v5-mss 2080000.qcom,mss: modem: modem.mdt starting auth and reset
pil-q6v5-mss 2080000.qcom,mss: modem: Brought out of reset
pil-q6v5-mss 2080000.qcom,mss: Subsystem error monitoring/handling services are up
pil-q6v5-mss 2080000.qcom,mss: modem: Power/Clock ready interrupt received
M-Notify: General: 7
IPA received MPSS AFTER_POWERUP
rmnet_ipa started initialization
IPA SSR support = True
IPA ipa-loaduC = True
IPA SG support = True
rmnet_ipa completed initialization
IPA AFTER_POWERUP handling is complete
subsys-restart: subsystem_restart_wq_func(): [ffffffc0f7c38b00]: Restart sequence for modem completed.
sysmon-qmi: sysmon_clnt_svc_arrive: Connection established between QMI handle and modem's SSCTL service
Sending QMI_IPA_INIT_MODEM_DRIVER_REQ_V01
ipa-wan handle_indication_req:133 not send indication
QMI_IPA_INIT_MODEM_DRIVER_REQ_V01 response received
ipa ipa_sps_irq_control_all:938 EP (5) not allocated.
ipa-wan ipa_check_qmi_response:384 Got bad response 49 from request id 1 (error 48)
ipa ipa_sps_irq_control_all:938 EP (5) not allocated.
ipa ipa_sps_irq_control_all:938 EP (5) not allocated.
Fatal error on the modem.
modem subsystem failure reason: lge_init_fw.c:221:Invalid lg_model_name(E1506CF6).
[LGE] MSM Chip version : 0, Family : 0, Id : 0
subsys-restart: subsystem_restart_dev(): Restart sequence requested for modem, restart_level = RELATED.
IPA received MPSS BEFORE_SHUTDOWN
ipa ipa_sps_irq_control_all:938 EP (5) not allocated.
rmnet_ipa started deinitialization
ipa ipa2_teardown_sys_pipe:1418 bad parm.
ipa-wan ipa_wwan_remove:2122 Failed to teardown IPA->APPS pipe
[RMNET:HI] rmnet_config_notify_cb(): Kernel is trying to unregister rmnet_ipa0
[RMNET:HI] rmnet_config_notify_cb(): Kernel is trying to unregister rmnet_ipa0
ipa-wan ipa_del_dflt_wan_rt_tables:377 Deleting Route hdl:(0x9) with ip type: 0
ipa-wan ipa_del_dflt_wan_rt_tables:386 Deleting Route hdl:(0xb) with ip type: 1
rmnet_ipa completed deinitialization
IPA BEFORE_SHUTDOWN handling is complete
subsys-restart: subsystem_shutdown(): [ffffffc0f8892100]: Shutting down modem
pil-q6v5-mss 2080000.qcom,mss: Port ffffff80022c4000 halt timeout
<CORE> smem:mpss glink_open: Error 0 - unable to find transport
<LBSRV> smem:mpss:LOOPBACK_CTL_APSS glink_lbsrv_open_worker: unable to open channel
IPA received MPSS AFTER_SHUTDOWN
ipa ipa_uc_reset_pipe:723 Invalid IPA client
ipa ipa_uc_reset_pipe:723 Invalid IPA client
ipa ipa_uc_reset_pipe:723 Invalid IPA client
ipa ipa_uc_reset_pipe:723 Invalid IPA client
ipa ipa_uc_reset_pipe:723 Invalid IPA client
ipa ipa_uc_reset_pipe:723 Invalid IPA client
IPA AFTER_SHUTDOWN handling is complete

The key lines appear to be:

ipa ipa_sps_irq_control_all:938 EP (5) not allocated.
ipa-wan ipa_check_qmi_response:384 Got bad response 49 from request id 1 (error 48)
modem subsystem failure reason: lge_init_fw.c:221:Invalid lg_model_name(E1506CF6).

The first is suspicious since that reads like the modem is attempting to communicate over a channel which was never opened. The second seems like the actual point where Linux kernel code is flagging that there is an error. The third looks like an actual extended error message.

Now I'm worried about the error message. This could be the modem on my H990DS got really badly screwed up during the course of my experimentation to get things working; in which case it is repair time. This could be the aboot stage does some amount of initialization of the modem, and since replacing that is required this is a battle that cannot be won.

One optimistic theory I've come up with is perhaps some portion of modem firmware loading is being done based on the contents of /sys/firmware/devicetree/base/chosen/bootargs, instead of /proc/cmdline. If so, this should be fixable by adjusting what that file contains, but that is going to be harder than modifying /proc/cmdline.

Alternatively I need to go back to full stock and see whether the modem starts working again (confirm the debug bootloader really is a problem for the modem).

The good news is very little of the hardware remains to be tested. Get the modem operational and we should have fully functional root. I think this is the last lap.