A Weird Imagination

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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Emotional error messages

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Cowardly tar

$ tar c
tar: Cowardly refusing to create an empty archive
Try 'tar --help' or 'tar --usage' for more information.

That's what the GNU implementation of tar says. For comparison, Debian includes FreeBSD's implementation of tar in the bsdtar package as the bsdtar command:

$ bsdtar c
bsdtar: no files or directories specified

Clingy lynx

In the text-only web browser lynx (not to be confused with links), if you press q to quit, it asks

Are you sure you want to quit? (y)

Most keys quit the program, but if you decide not to quit and press n for no, lynx shows the message

Excellent!!!

which goes away after a couple seconds allowing you to continue browsing.

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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Shadowrun's text compression

The problem

Several years ago, I was in a ROM hacking IRC room where another regular Alchemic was reverse engineering the text system of the SNES game Shadowrun. He figured it out and wrote a python script to decompress the text but had some questions about why it was designed the way it was. So we're going to walk through figuring out how the code works, with some help from his notes, and try to understand the design.

If you don't want spoilers and would rather try to reverse engineer it yourself, just read up to the end of the Trace format section and see how much you can figure out on your own.

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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.

How not to fix USB HID errors

The error

I was having trouble with a USB joystick adapter (an EMS Playstation controller adapter, to be specific). When I plugged it in, it wouldn't work and checking dmesg showed the same error getting generated over and over again (at least once per second):

$ dmesg
...
[81700.968873] usbhid 6-1:1.0: can't add hid device: -71
[81700.968885] usbhid: probe of 6-1:1.0 failed with error -71
[81700.968986] usb usb6-port1: disabled by hub (EMI?), re-enabling...
[81700.968991] usb 6-1: USB disconnect, device number 53
[81701.208025] usb 6-1: new low-speed USB device number 54 using uhci_hcd
[81701.384866] usb 6-1: string descriptor 0 read error: -32
...

The wrong fix

I decided the sensible thing to do was to reload the driver:

$ sudo modprobe -r usbhid  # Bad idea, don't run this

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