The stuff in sound/i2c/* are mostly for the i2c bus on a PCI sound cards, an implementation independent from the standard i2c stuff.
And, in your case, it's hard to know how the i2c bus is connected. If it's controlled over HD-audio GPIO pin (one for clk and one for data), then some stuff in sound/i2c can be re-used. Or, if it's on another i2c bus, the story will be completely different...
I have the schematics of the CS8409. the i2c is connected to the GPIO of the HDA.
MAX98374 has a Reset Low pin which is connected to GPIO 5 on the HDA controller CS8409. This must be driven high, then wait for 1.5 ms. Only then will the amps be out of device shutdown. GPIO6 is 8409_I2C_SCL GPIO7 is 8409_I2C_SDA
GPIO5 is AUD_SPKRAMP_RESET_L
Knowing GPIO6 is the clock and GPIO7 is the data, how can I use them? Can I use hda-verb /dev/snd/hwC0D0 0x01 XXX YYY ? what XXX YYY would be ?