A Weird Imagination

Lightweight multiseat X

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The problem

I hosted a LAN party1 a little while ago and ended up needing to loan out multiple computers to guests in the interest of having no one try to lug their desktop over. As it turns out, I don't keep multiple of spare gaming-ready laptops around, so I needed to get more computers somehow.

The solution

My desktop has three screens attached to it (two monitors plus a projector), so given an extra keyboard and mouse (or two), it should be possible to run multiple instances of the game on it at the same time to let multiple people play using the same computer.

The script from this forum post makes it easy to set up multi-pointer X so a second keyboard and mouse will get its own mouse cursor. Then each keyboard and mouse pair can interact with its own instance of the game.

As an additional aid, I wrote monitor-lock.py which allows you to assign a mouse to a monitor, so it cannot be moved off that monitor to prevent accidentally interacting with the other player's instance of the game.

The basic usage is that you first run it with no arguments to get the available screens and pointers getting an output something like this:

$ ./monitor-lock.py 
...
Available screens:
screen 0: {'x': 0, 'y': 0, 'width': 3840, 'height': 2160}
screen 1: {'x': 3840, 'y': 0, 'width': 1920, 'height': 1200}
screen 2: {'x': 3840, 'y': 1200, 'width': 1920, 'height': 1080}

Available pointers:
device 2: Virtual core pointer
device 17: second pointer

USAGE: ./monitor-lock.py [device] [screen]

and then in a screen session (so you don't have to worry about accidentally doing this on a monitor you've locked your pointer away from), run

./monitor-lock.py 2 0

and

./monitor-lock.py 17 1

to lock the primary pointer to the first screen and the second pointer to the second screen.

Just use Ctrl+C to kill the process when you want the pointer to be able to move freely again.

The details

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Emulating Xbox controllers using GameCube controllers

The problem

I previously wrote about making different controllers act like Xbox 360 controllers. While it's a useful general-purpose solution, it's can be a bit clunky to have to explicitly set the mappings for each controller. More importantly, the remapping leaves the original controller entries in /dev/input/, although they don't do anything, and some games1 assume that the four players are controlled by the first four controllers. This is no longer true if js0 is the real first controller and js1 is the copy made by xboxdrv to look like an Xbox 360 controller. Or, worse, if js0-js3 are the four real controllers and js4-js7 are the ones we want the game to actually use.

The specific reason I'm remapping the controllers, is that the gamepads I'm actually using are GameCube controllers connected via the Nintendo GameCube controller Adapter for Wii U, which connects up to four GameCube controllers to a USB port. wii-u-gc-adapter makes them usable as controllers, but they appear different enough from Xbox 360 controllers that remapping them is necessary for most games.

The solution

Just build and use the version of wii-u-gc-adapter in my feature/mimic-xpad branch and your GameCube controllers will show up as Xbox controllers.

The details

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Encrypted files in Vim

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The problem

There's a handy Vim plugin openssl.vim that allows you to easily edit encrypted files with Vim simply by giving the file an extension like .aes. Then Vim will ask for a password upon loading and saving the file in order to decrypt and encrypt it with openssl.

Unfortunately, the plugin was last updated in 2008 and makes some assumptions about openssl's defaults which are no longer valid. The most pressing issue is that the plugin now outputs a warning message when encrypting. By itself, that's worrisome, but, worse, that warning message gets output into the file along with the ciphertext. Needless to say, the resulting file cannot be decrypted without manually removing the warning text.

The solution

Simply fixing the options the script passes to openssl is a good start, but I also wanted to make sure any files encrypted with the old settings could be decrypted. My updated openssl.vim1 does both in addition to fixing some other annoyances.

The details

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PulseAudio headphone jack troubles

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The problem

Since I got a new motherboard (and therefore new audio hardware as I'm using the basic one built into the motherboard) sometimes after I unplugged my headphones, my speakers would not output any sound.

pavucontrol showed the only available output as "Built-in Audio Digital Stereo" with a port of "S/PDIF", which does not describe any audio device I had ever used. If I plugged my headphones back in, they would work fine, and usually after unplugging and plugging back in my headphones enough times, my computer would eventually acknowledge that my speakers were connected by showing the expected "Built-in Audio Analog Stereo" with a port of "Line Out".

The solution

In /usr/share/pulseaudio/alsa-mixer/paths/analog-output-lineout.conf change

[Jack Front Headphone]
state.plugged = no
state.unplugged = unknown

to

[Jack Front Headphone]
state.plugged = no
state.unplugged = yes       # changed from unknown

This forces PulseAudio to consider there to be speakers plugged into the "Line Out" port, so it may cause strange behavior if that is not the case.

To apply the change, run

pulseaudio --kill
pulseaudio --start

to restart PulseAudio.

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Nvidia GLX not working

The problem

I recently replaced my old Nvidia graphics card with a newer one. Upon booting up, I ran glxgears to test that 3D graphics were working properly and got an error like

X Error of failed request:  BadWindow (invalid Window parameter)
 Major opcode of failed request:  155 (NV-GLX)
 Minor opcode of failed request:  4 ()
 Resource id in failed request:  0x1200003
 Serial number of failed request:  34
 Current serial number in output stream:  34

The solution

Either delete /etc/X11/xorg.conf or edit it and remove (or comment out) the "Files" section; that is, the lines

Section "Files"
    ...
EndSection

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Volume via shell

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The problem

Sometimes a GUI is not the best way to control a computer's volume. Usually if you care about the volume of your computer, you're probably nearby but perhaps would rather be using a remote or other shortcut way of changing the volume. The specific use case that prompted this blog post was binding the volume up and volume down keys on my keyboard to the global volume control (as opposed to separately binding them in each application).

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Application bypassing PulseAudio

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The problem

Recently I ran a game1 and instead of the expected music, got distorted noise. At first I thought there was something physically wrong with my speakers or the connection to them, but running any other program resulted in normal sound, albeit mixed with the distorted sound of the game. Even more strangely, changing the volume in the game changed the volume of the distorted noise, implying the game was in fact generating the right thing but it was being misinterpreted, so the culprit was neither the game nor the sound driver but somewhere in between them.

As I had recently set up PulseAudio2, I suspected it was to blame. I opened up pavucontrol to find the game omitted from the list of applications producing sound, which suggested the problem was caused by the game trying to use some way to produce sound that PulseAudio was not capturing.

The solution

The short version is that the problem was solved by restarting PulseAudio:

$ killall -9 pulseaudio

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Emulating Xbox controllers on Linux

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The problem

The Xbox 360 controller has become the defacto standard controller in PC gaming in recent years, likely due to both the popularity of the Xbox and the fact that the controller can easily be used with a computer. One downside of this is that some games assume you have one. If the game supports it and is running through Steam, then Steam's controller settings will let you use any controller, but that doesn't work for all games, and you might not be using Steam. The game that prompted this blog post actually does have Steam controller support promised in the future, but it's in early access and they are busy developing other parts of the game.1

xboxdrv

The solution is xboxdrv, the userspace Xbox controller driver. In addition to supporting actual Xbox controllers, it can also simulate Xbox controllers based on inputs from other devices like a PlayStation controller or some less common controller.

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Identifying joystick devices

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Too many input devices

On a modern computer there are often many input devices,

$ ls /dev/input/event* | wc -l
28

They are just identified by numbers, so it can be difficult to choose the right one and trial-and-error can get tiresome with so many. There is some help from the by-id and by-path listings:

$ ls -go --time-style=+ /dev/input/by-id/
...
lrwxrwxrwx 1 10  usb-045e_0291-if06-event-joystick -> ../event26
lrwxrwxrwx 1  6  usb-045e_0291-if06-joystick -> ../js6
lrwxrwxrwx 1 10  usb-0b43_0003-event-if00 -> ../event20
lrwxrwxrwx 1 10  usb-0b43_0003-event-joystick -> ../event19
lrwxrwxrwx 1  6  usb-0b43_0003-joystick -> ../js1
lrwxrwxrwx 1  9  usb-BTC_USB_Multimedia_Keyboard-event-if01 -> ../event2
lrwxrwxrwx 1  9  usb-BTC_USB_Multimedia_Keyboard-event-kbd -> ../event1
...

$ ls -go --time-style=+ /dev/input/by-path/
...
lrwxrwxrwx 1  9  pci-0000:00:1a.2-usb-0:2:1.0-event-kbd -> ../event1
lrwxrwxrwx 1  9  pci-0000:00:1a.2-usb-0:2:1.1-event -> ../event2
...
lrwxrwxrwx 1 10  pci-0000:00:1d.0-usb-0:1:1.6-event-joystick -> ../event26
lrwxrwxrwx 1  6  pci-0000:00:1d.0-usb-0:1:1.6-joystick -> ../js6
lrwxrwxrwx 1 10  pci-0000:00:1d.0-usb-0:2:1.0-event -> ../event20
lrwxrwxrwx 1 10  pci-0000:00:1d.0-usb-0:2:1.0-event-joystick -> ../event19
...

But, for the most part, those names aren't very helpful, especially since many joystick devices support 2 or 4 joysticks connected to the same device.

identify_evdev.py

Enter identify_evdev.py:

$ identify_evdev.py
/dev/input/event22

Where /dev/input/event22 is the device of the joystick I touched after running identify_evdev.py.

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Read-only filesystem errors

Linux has a tendency to give very unhelpful error messages when it is unable to create a file. I previously blogged about a few different reasons Linux might report a disk is full, but all of the reasons included the disk actually not having space for more files. Yet another reason to get similar errors is if the partition is mounted readonly (ro):

$ mount | grep -F /usr
/dev/sdc2 on /usr type ext4 (ro,nodev,noatime,data=ordered)

mount without any options lists all of the mounted partitions along with their mount options.

Many programs will show a helpful error message:

$ touch test
touch: cannot touch ‘test’: Read-only file system

But some others won't:

rtorrent: Could not lock session directory: "./session/", held by "<error>".

That error is normally caused by ./session/rtorrent.lock not being writable due to being held by another process, but in this case it's not writable due to the filesystem being readonly. rtorrent doesn't distinguish the two.

For that reason, when running into weird behavior from a program on Linux, it's a good idea to check that the directories the program might try to write to are actually writable.