(Linux/Ubuntu) Frequent 802.11g/WPA2 connection drops with brcmsmac/BCM4313 driver

My Samsung laptop running Ubuntu 14.10 has been dropping its WiFi connection a lot. A cursory examination of the system log reveals that it usually happens once every 20-40 minutes, with a few suspicious cases where it dropped and re-established the connection after exactly 25 minutes:

$ grep "wlan0: associated" /var/log/syslog.1
Mar 17 20:45:48 gravitycat kernel: [101379.946156] wlan0: associated
Mar 17 21:10:48 gravitycat kernel: [102880.685412] wlan0: associated
Mar 17 21:33:32 gravitycat kernel: [104244.524634] wlan0: associated
Mar 17 21:43:03 gravitycat kernel: [104815.501396] wlan0: associated
Mar 17 22:08:03 gravitycat kernel: [106316.384463] wlan0: associated
Mar 17 22:11:23 gravitycat kernel: [   35.255877] wlan0: associated
Mar 17 22:46:55 gravitycat kernel: [ 2167.317562] wlan0: associated
[...]
Mar 17 23:29:08 gravitycat kernel: [ 4701.239455] wlan0: associated
Mar 17 23:54:08 gravitycat kernel: [ 6201.802957] wlan0: associated
[...]
Mar 18 00:00:26 gravitycat kernel: [ 6579.424244] wlan0: associated
Mar 18 00:25:26 gravitycat kernel: [ 8080.034656] wlan0: associated

The connection drops usually looked like this:

Mar 17 22:08:03 gravitycat NetworkManager[785]:  Connection disconnected (reason -4)

But were sometimes preceded by this:

Mar 18 01:09:43 gravitycat kernel: [10736.798220] wlan0: cannot understand ECSA IE operating class 33, disconnecting

It alternates between "class 33" and "class 12". After a tiny bit of digging, I found a suggestion here to try switching from Gnome's NetworkManager to WICD. At first, that seemed to make things better, but it started happening again after about an hour, with the same really frustrating behaviour (having to repeatedly queue the same videos on my Chromecast when it sees me "leave" due to the laptop dropping and reconnecting to the router).

Next port of call is to check the driver. Here's what lspci says is installed in my laptop:

$ lspci -vvnn | grep -A 9 Network 
03:00.0 Network controller [0280]: Broadcom Corporation BCM4313 802.11bgn Wireless Network Adapter [14e4:4727] (rev 01)
 Subsystem: Askey Computer Corp. Device [144f:7179]

For this device, Ubuntu defaults to the "brcmsmac" open source driver, which is always preferable to a closed-source proprietary driver, all other things being equal. This issue is so frustrating, however, that I'll take the closed-source "bcmwl" driver if it works, following these steps:

$ sudo apt-get install bcmwl-kernel-source
$ sudo modprobe -r brcmsmac bcma
$ sudo modprobe wl

After manually reconnecting with WICD, things seem stable and the ChromeCast hasn't bailed over the past day or so. It's a shame the open source driver isn't working well with the BCM4313 chipset. It'd be nice if there was a simple way to get started debugging/testing the driver – i.e. not having to download a massive tarball of the latest kernel source and spend hours tweaking with build scripts and the like. Unless things have changed in the last few years and now this is easy...?

OpenCV 2.4.10 and Macports

Every time I've needed to build OpenCV on my work Mac, it's been a bit of a struggle. I have Macports installed which helps make some things easier, but throws up problems of its own:
(using "mp-gcc49")

/Users/oisin/Documents/code/vision/opencv-2.4.10/modules/highgui/src/window_cocoa.mm: In function 'void cvDestroyAllWindows()':
/Users/oisin/Documents/code/vision/opencv-2.4.10/modules/highgui/src/window_cocoa.mm:202:23: error: expected ';' before 'in'
     for(NSString *key in list) {

(using "apple-gcc42")

/System/Library/Frameworks/QTKit.framework/Headers/QTMovieModernizer.h:123:46: error: expected ')' before '(' token
- (void)modernizeWithCompletionHandler:(void (^)(void))handler AVAILABLE_QTKIT_VERSION_7_7_3_AND_LATER;
                                          ^
/System/Library/Frameworks/QTKit.framework/Headers/QTMovieModernizer.h:123:46: error: expected identifier before '(' token

The problem appears to be Objective-C stuff in QTKit and in OpenCV's HighGUI code. This time, the solution was to revert to the system-default C/C++ compiler, Clang:

$ sudo port select gcc none
Warning: port definitions are more than two weeks old, consider updating them by running 'port selfupdate'.
Selecting 'none' for 'gcc' succeeded. 'none' is now active.

It's slightly counterintuitive that "port select gcc none" is how you go back to the system compiler with Macports, and it doesn't appear to be stated in the docs, so... well here it is!