[ltp] Re: spotty wireless with t520 w/Intel wifi and xubuntu 13.04

Henrique de Moraes Holschuh linux-thinkpad@linux-thinkpad.org
Mon, 28 Oct 2013 13:48:50 -0200

On Sat, 26 Oct 2013, Michael Livshin wrote:
> Where was I?  Ah, yes, NetworkManager.  See, that fine piece of software
> is periodically (as in every fscking two minutes) issuing AP scans.


> Now, it looks like many Linux wireless drivers (or perhaps it's a deeper
> kernel WiFi infrastructure problem?  Apparently nobody has a real clue!)

Depends on the driver.  Some use the unified software-MAC code, others use
driver-specific code.  Older chipsets did the MAC layer in firmware (e.g.
ipw2200 from the 'Intel Centrino' era).

However, if you DO specify which driver you're using, people at
linux-wireless do know exactly where the background scanning is implemented,
and how.

> have trouble keeping a connection up and scanning at the same time,
> which trouble manifests itself by disconnection from the active AP and,
> in many cases (like with this here RTL8192CE), ceasing to see that AP
> for some time (usually, but not always, cured by reloading the driver
> module).  The problem is greatly exacerbated by poor (60% or less)
> signal in the first place.

This is clearly a kernel bug (or several), and it should be reported.  When
fixed, you should also keep notice of *where* it was fixed, since fixes for
this kind of stuff are not always backportable to the stable kernels...

> The NetworkManager maintainers are refusing to introduce a configuration
> knob to turn those moronic periodic scans off, because they figure that

And this is clearly the kind of crap that makes NM and most things Gnome a
real pain in the ass.  Even when properly implemented in the driver,
background scans *WILL* introduce large jitter spikes/latency spikes and
thus you might well want to disable it for that reason alone.

  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh