In the existing IPC support, the reply to each IPC message is handled in
an IRQ thread. The assumption is that the IRQ thread is scheduled without
significant delays.

On an experimental (iow, buggy) kernel, the IRQ thread dealing with the
reply to the last IPC message before powering-down the DSP can be delayed
by several seconds. The IRQ thread will proceed with register accesses
after the DSP is powered-down which results in a kernel crash.

While the bug which causes the delay is not in the audio stack, we must
handle such cases with defensive programming to avoid such crashes.

Call synchronize_irq() before proceeding to power down the DSP to make
sure that no irq thread is pending execution.

Closes: https://github.com/thesofproject/linux/issues/4608Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20231012191850.147140-3-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>