SAI or SPI clock requirement as I2S slave

When acting as I2S master, the kernel clock frequency for SAI or SPI must be set to some multiple of the audio sample rate, and the bit clock and frame sync signals are then generated by dividing the kernel clock. Now, if acting as I2S slave the bit clock and frame sync signals are inputs, is the kernel clock still needed in this case? And if yes, does it have to be synchronous to the bit clock and frame sync signals which are supplied by an external device?

The user manual does not say anything about the kernel clock requirement in slave mode.