"In het verleden behaalde resultaten bieden geen garanties voor de toekomst"

These are the ramblings of Matthijs Kooijman, concerning the software he hacks on, hobbies he has and occasionally his personal life.

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My old blog (pre-2006) is also still available.

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Thinkpad X201 mute button breaking speaker output

Recently, I was having some problems with the internal speakers on my Lenovo Thinkpad X201. Three times now, the internal speakers just stopped producing sound. The headphone jack worked, it's just the speakers which were silent. Nothing helped: fiddling with volume controls, reloading alsa modules, rebooting my laptop, nothing fixed the sound...

When trying to see if the speakers weren't physically broken, I discovered that booting into Windows actually fixed the problem and restored the sound from the speakers. It's of course a bit of a defeat to accept Windows a fix for my problem, but I was busy with other things, so it sufficed for a while.

When migrating my laptop to my new Intel SSD, I broke my Windows installation, so when the problem occured again, I had no choice but to actualy investigate it.

I'll skip right to the conclusion here: I had broken my sound by pressing the mute button on my keyboard... Now, before you think I'm stupid, I had of course checked my volume controls and the device really was unmuted! But it turns out the mute button in Thinkpads combined with Linux is a bit weird...

This is how you would expect a mute button to be implemented: You press the mute button, it sends a keypress to the operating system, which then tells the audio driver to mute.

This is how it works on my Thinkpad: You press the mute button, causing the EC (embedded controller) in the thinkpad to directly mute the speakers. This is not visible from the normal volume controls in the software, since it happens on a very low level (though the thinkpad_acpi kernel module can be used to expose this special mute state through a /proc interface and special audio device).

In addition to muting the speakers, it also sends a MUTE acpi keypress to the operating system. This keypress then causes the audio driver to mute the audio stream (actually, it's pulseaudio that does that).

Now, here's the fun part: If you now unmute the audio stream through the software volume controls, everything looks like it should work, but the hardware is still muted! It never occured to me to press the mute button again, since the volume wasn't muted (or at least didn't look like it).

I originally thought that the mute button handling was even more complex, when I found some register polling code that faked keypresses, but it seems that's only for older Thinkpads (phew!).

In any case, the bottom line is: If you have a Thinkpad whose speakers suddely stop working, try pressing the mute button!