ALSA: control - add generic LED API This patchset tries to resolve the diversity in the audio LED control among the ALSA drivers. A new control layer registration is introduced which allows to run additional operations on top of the elementary ALSA sound controls. A new control access group (three bits in the access flags) was introduced to carry the LED group information for the sound controls. The low-level sound drivers can just mark those controls using this access group. This information is not exported to the user space, but user space can manage the LED sound control associations through sysfs (last patch) per Mark's request. It makes things fully configurable in the kernel and user space (UCM). The actual state ('route') evaluation is really easy (the minimal value check for all channels / controls / cards). If there's more complicated logic for a given hardware, the card driver may eventually export a new read-only sound control for the LED group and do the logic itself. The new LED trigger control code is completely separated and possibly optional (there's no symbol dependency). The full code separation allows eventually to move this LED trigger control to the user space in future. Actually it replaces the already present functionality in the kernel space (HDA drivers) and allows a quick adoption for the recent hardware (ASoC codecs including SoundWire). snd_ctl_led 24576 0 The sound driver implementation is really easy: 1) call snd_ctl_led_request() when control LED layer should be automatically activated / it calls module_request("snd-ctl-led") on demand / 2) mark all related kcontrols with SNDRV_CTL_ELEM_ACCESS_SPK_LED or SNDRV_CTL_ELEM_ACCESS_MIC_LED Link: https://lore.kernel.org/r/20210317172945.842280-1-perex@perex.cz Signed-off-by: Takashi Iwai <tiwai@suse.de>