when using a kernel with IRQ handlers (e.g. patched with preempt_rt) it is advisable to elevate the priorities of all IRQ handler threads for the soundcard to above the other IRQ handlers. I am not sure though which IRQ handlers those are for the hifiberry dacplusadcpro on a rpi 4b.
My current guess are IRQs 23 and 24 (which are named "irq/23-DMA IRQ" and "irq/24-DMA IRQ" respectively).
Are these correct?