Modemmanager significant startup delay

Bjørn Mork bjorn at
Thu Aug 22 09:06:27 UTC 2019

Dan Williams <dcbw at> writes:
> On Wed, 2019-08-21 at 12:23 +0000, Amol Lad wrote:
>> Please help with this. What could be the cause of significant MM
>> startup delay?
> When started, or when a new modem is plugged attached, ModemManager
> runs through a hardware detection sequence to figure out whether the
> thing you attached is actually a modem and what ports it has and what
> protocols those ports speak.

Not prepared to write the code, so I should probably not make
suggestions...  But do anyway ;-)

How about caching protocol probe results and skipping the full probe if
a match is found?

Keying the cached data and knowing when to ignore the cache is probably
the hard part.  But I believe there is a lot to gain for normal use
cases.  Most people don't use lots of modems.  Many systems will have
the same modem connected to the same port for extended periods. Some
system designs cause the MM probing to run very often, "always"
returning the same result. Looking at my laptop for example: It turns
off power to the internal wwan slot on suspend. So MM must probe the
modem on every resume, causing a significant delay from the system
appears to be up until the network is up again.

The protocol probes are obviously a waste of time in this case. Unless I
am messing with drivers or descriptors :-) But that's easy to detect.
You could just ignore the protocol cache if any of the udev data
changed.  It would still be a large win for most cases.


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