https://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html

After application transfers the data in the memory areas, then it must be acknowledged the end of transfer via snd_pcm_mmap_commit() function to allow the ALSA library update the pointers to ring buffer. This kind of communication is also called “zero-copy”, because the device does not require to copy the samples from application to another place in system memory.

When you tell RTFM expect to see manual stating opposite of your point.