6:09 p.m.
Hi Harald,
On 07/11/2016 12:37 PM, Harald Welte wrote:
Dear all,
I've been struggling for quite some time to find out what is the
designated way in ofono to deal with modems that actually perform a
cold/warm reset (and hence USB device stack reset) when you send them an
AT+CFUN=1 message.
The best way to deal with this is not to have the modem reset :) Does
Sierra Wireless have any extensions to turn off this behavior?
Examples for this are e.g. the Sierra Wireless SL808x series of modems,
where this behavior is documented and expected. But I'm sure there are
others, at least I recall having seen this several times in the past.
Generally this isn't done by the modems. Some will physically power off
when you send them a CFUN=0. Then you'd need to replug the USB cable.
It is somewhat logical, as a +CFUN+1 is supposed to reset _all_ of the
status in the modem, not just the protocol stack. So the fact that most
modems don't disappear from the bus afterwards actually means that they
are cheating.
No, it means they are sane. Many modem manufacturers use a second
parameter to CFUN, which tells the modem whether to reset or not. E.g.
CFUN=1,1 to cause a full reset.
The actual behavior of ofono with such a modem is:
* modem first appears as a /wavecom_X device
* if you issue enable_modem via dbus, the modem goes thrugh CFUN=1
* the modem disappears from USB, ofono destroys its structures
* the client program (e.g. the one from tests/) simply hangs and fails
at timeout
* the modem re-enumerates on the bus and ofono creates a new /wavecom_Y
device, where Y != the X before the modem restart.
So for an application it is not possible to continue to talk to one
modem via dbus accross modem resets.
Not issuing a +CFUN=1 is also not really an option, as then you loose
the ability to completely disable the modem. +CFUN=4 can be used to
force an IMSI detach and disable it from the network point of view, but
the modem is still talking to the SIM card and will not re-read SIM
card state if brought back online from +CFUN=4.
I know of no USB based device that provides a complete power down
setting. Some (like Ericsson) just jump off the USB bus when a CFUN=0
is issued, but it doesn't really help, as you'd need to physically take
the modem out and plug it back in. Hence we just go back to CFUN=4 mode
on disable for these devices.
Since most of these devices need to be pulled out in order to change the
SIM card, it is generally not a problem.
I'm sure this issue has been seen before, and somebody must have a
solution for it - or at least some ideas on how to solve this in a ofono
driver plugin. I'm willing to spend some time on it, but my
experience/exposure to the code base is limited.
This is just a limitation of USB devices. If you want proper control of
a modem power state, you might want to research devices with
full-featured modems on board.
Regards,
-Denis