Public bug reported:
This is an interesting bug that is triggered under some circumstances if
one of the official, MediaTek-SoC-based Ubuntu phones is connected to a
Linux host via USB.
Tested host:
Ubuntu 15.04 Desktop, kernel 3.19.0-21-generic, package linux-image-generic
3.19.0.22.21 amd64, package modemmanager 1.4.0-1
Tested phones:
Meizu MX4 (arale) and bq Aquaris E4.5 (krillin), image version irrelevant
because the Preloader comes with all devices and cannot be changed
How to reproduce, possibility 1:
1. Disconnect the USB connection, if necessary.
2. Turn off the phone.
3. Connect the phone via USB. It will take a bit and then turn on, displaying
the "charge animation".
How to reproduce, possibility 2:
1. Reboot the phone while it is still attached to the host.
The kernel ringbuffer will contain the following log messages (example
for krillin):
[288774.244158] usb 1-1: new high-speed USB device number 22 using xhci_hcd
[288774.373589] usb 1-1: New USB device found, idVendor=0e8d, idProduct=2000
[288774.373593] usb 1-1: New USB device strings: Mfr=1, Product=2,
SerialNumber=0
[288774.373594] usb 1-1: Product: MT65xx Preloader
[288774.373596] usb 1-1: Manufacturer: MediaTek
[288774.435247] cdc_acm 1-1:1.1: ttyACM26: USB ACM device
[288774.874607] usb 1-1: USB disconnect, device number 22
[288774.874755] cdc_acm 1-1:1.1: failed to set dtr/rts
And syslog will contain the following log messages:
Jul 10 00:58:07 fire ModemManager[1103]: <info> (tty/ttyACM27): released by
modem /sys/devices/pci0000:00/0000:00:14.0/usb1/1-1
Jul 10 00:58:10 fire ModemManager[1103]: <warn> (Plugin Manager) (Iridium)
[ttyACM27] error when checking support: '(Iridium) Missing port probe for port
(tty/ttyACM27)'
Jul 10 00:58:10 fire ModemManager[1103]: <warn> (Plugin Manager) (Cinterion)
[ttyACM27] error when checking support: '(Cinterion) Missing port probe for
port (tty/ttyACM27)'
Jul 10 00:58:10 fire ModemManager[1103]: <warn> (Plugin Manager) (Nokia)
[ttyACM27] error when checking support: '(Nokia) Missing port probe for port
(tty/ttyACM27)'
Jul 10 00:58:10 fire ModemManager[1103]: <warn> (Plugin Manager) (Generic)
[ttyACM27] error when checking support: '(Generic) Missing port probe for port
(tty/ttyACM27)'
Jul 10 00:58:10 fire ModemManager[1103]: <warn> Couldn't find support for
device at '/sys/devices/pci0000:00/0000:00:14.0/usb1/1-1': not supported by any
plugin
The allocated ttyACMx device is never freed, and after a couple of days
developing on the phone (with lots of phone restarts) I've noticed that
my machine has accumulated an usage count of 58:
$ lsmod | grep cdc_acm
cdc_acm 36864 58
lsof show that ModemManager still holds the devices:
# sudo lsof | grep ACM
ModemMana 9214 root 9u CHR 166,1
0t0 7599355 /dev/ttyACM1 (deleted)
ModemMana 9214 root 10u CHR 166,0
0t0 7596974 /dev/ttyACM0 (deleted)
...
Restarting ModemManager immediately reduces the usage count to 0 again
and frees all devices.
Why does this happen? Luckily I've read the MediaTek Preloader source
code. The Preloader is the first-stage bootloader on MediaTek SoCs and
exposes a proprietary USB interface that can be used for debugging
purposes and to download firmware to the device. The implementation of
this interface is rudimentary and emulates a CDC ACM serial port, which
probably doesn't fully conform to the standard and also just falls from
the bus after a short amount of time if the host doesn't initiate the
proprietary protocol. So I think the kernel and ModemManager immediately
try to use the emulated device when it becomes available, but because
the implementation in the Preloader is incomplete some bug (timeout?) is
triggered and ModemManager never closes the device.
Sadly I don't have the expertise to decide if this is a bug in
ModemManager or the kernel or both, so I file it with ModemManager.
** Affects: modemmanager (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1473246
Title:
MediaTek Preloader on a mobile device triggers cdc_acm kernel module
usage by ModemManager
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