netdev - imx7: USB modem reset causes modem to not re-connect

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Date:   Mon, 12 Dec 2022 15:10:56 -0300
From:   Fabio Estevam <festevam@...il.com>
To:     bjorn@...k.no, Peter Chen <peter.chen@...nel.org>,
        Marek Vasut <marex@...x.de>, Li Jun <jun.li@....com>
Cc:     netdev <netdev@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Alexander Stein <alexander.stein@...tq-group.com>,
        Schrempf Frieder <frieder.schrempf@...tron.de>
Subject: imx7: USB modem reset causes modem to not re-connect

Hi,

On an imx7d-based board running kernel 5.10.158, I noticed that a
Quectel BG96 modem is gone after sending a reset command via AT:

# microcom /dev/ttyUSB3
>AT+CFUN=1,1
OK
 usb 2-1: USB disconnect, device number 6
option1 ttyUSB0: GSM modem (1-port) converter now disconnected from ttyUSB0
option 2-1:1.0: device disconnected
option1 ttyUSB1: GSM modem (1-port) converter now disconnected from ttyUSB1
option 2-1:1.1: device disconnected
option1 ttyUSB2: GSM modem (1-port) converter now disconnected from ttyUSB2
option 2-1:1.2: device disconnected
option1 ttyUSB3: GSM modem (1-port) converter now disconnected from ttyUSB3
option 2-1:1.3: device disconnected
qmi_wwan 2-1:1.4 wwan0: unregister 'qmi_wwan' usb-ci_hdrc.1-1, WWAN/QMI device

# lsusb
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

The USB modem is gone.

Forcing an 'echo on' to power/control makes the USB modem reappear:

# echo 'on' > /sys/bus/usb/devices/usb2/power/control
usb 2-1: new high-speed USB device number 7 using ci_hdrc
 usb 2-1: New USB device found, idVendor=2c7c, idProduct=0296, bcdDevice= 0.00
 usb 2-1: New USB device strings: Mfr=3, Product=2, SerialNumber=4
usb 2-1: Product: Qualcomm CDMA Technologies MSM
 usb 2-1: Manufacturer: Qualcomm, Incorporated
usb 2-1: SerialNumber: 7d1563c1
option 2-1:1.0: GSM modem (1-port) converter detected
usb 2-1: GSM modem (1-port) converter now attached to ttyUSB0
 option 2-1:1.1: GSM modem (1-port) converter detected
usb 2-1: GSM modem (1-port) converter now attached to ttyUSB1
option 2-1:1.2: GSM modem (1-port) converter detected
 usb 2-1: GSM modem (1-port) converter now attached to ttyUSB2
option 2-1:1.3: GSM modem (1-port) converter detected
usb 2-1: GSM modem (1-port) converter now attached to ttyUSB3
qmi_wwan 2-1:1.4: cdc-wdm0: USB WDM device
qmi_wwan 2-1:1.4 wwan0: register 'qmi_wwan' at usb-ci_hdrc.1-1,
WWAN/QMI device, 12:bc:8c:zz:yy:xx

# lsusb
Bus 002 Device 007: ID 2c7c:0296 Quectel Wireless Solutions Co., Ltd.
BG96 CAT-M1/NB-IoT modem
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Sending the AT reset command afterward works fine, and the modem keeps
connected.

Previously, this board used a vendor-based 4.14 kernel and such a
problem did not happen.

Kernels 5.10 and 4.14 have the same 'auto' option selected by default.

Also tested kernel 6.1 and it behaves the same as 5.10.158.

What can be done so that the reset modem command does not cause the
modem to disappear by default?

Thanks,

Fabio Estevam