Fex

I found a workaround! After looking at some kernel stack traces, I found that this guy was responsible: https://www.kernel.org/doc/html/latest/sound/soc/dapm.html When the PCM device is closed the following happens in the kernel: snd_pcm_release() -> soc_pcm_close() -> delay of `pmdown_time` -> close_delayed_work() The last function powers down the audio subsystem, causing the hum. To circumvent this behavior, I simply set `pmdown_time` to -1: echo -1 > /sys/devices/platform/soc@01c00000/1c22c00.codec/cdc/pmdown_time Now, once the sound subsystem is powered on, it stays powered on. Not ideal but it seems to work.

Thanks for the tip! Still no luck :-( I changed the governor from "ondemand" to "performance", but the problem persists. I also noticed that the hum stays during playback, if either "Power Amplifier DAC" or "Power Amplifier Mute" is muted.

After figuring out how to enable analog audio on the Lime2 (https://forum.armbian.com/index.php?/topic/4224-analog-audio-on-lime2-with-stable-kernel-version-49/), I now have the problem that there is a clearly noticeable mainline hum whenever no sound is playing. The hum goes away as soon as any sound is playing on the device, and it returns a couple seconds after the sound stopped. It appears like some kind of power saving feature kicks in. With the legacy kernel the hum goes away during boot and does not return. Any ideas?

I found the solution: Unmute "Power Amplifier DAC" and "Power Amplifier Mute". Why those are muted per default is beyond me :-) See http://linux-sunxi.org/Audio_Codec

While analog audio output (through the GPIO connector) works out of the box using the legacy kernel 3.4.113-sun7i, I could not get it to work with the stable kernel 4.9.12-sunxi. Since the driver is not activated in the stable kernel, I added the following to the dts: &codec { status = "okay"; }; This lead to the following `dmesg` output: sun4i-codec 1c22c00.codec: Codec <-> 1c22c00.codec mapping ok `aplay -l` gives: **** List of PLAYBACK Hardware Devices **** card 0: sun4icodec [sun4i-codec], device 0: CDC PCM Codec-0 [] Subdevices: 1/1 Subdevice #0: subdevice #0 However when I play back some audio file, no sound is generated. Also using the legacy kernel, there is a particular point during boot time, when the power line hum goes away. In the stable kernel this does not happen; the hum stays. Does anybody know to fix this?