STM32 gotchas
178.I2SxExt_SD and SPIx_MISO are different AF in 'F4, but the same AF in 'F3

As discussed previously, the I2SxExt modules are stripped-down slave-I2S-only versions of the SPI/I2S module, nonetheless they are separate modules connected to their "master" SPIx/I2Sx counterpart only at the IO-signals level (internal to the chip, but external to modules themselves). In the 'F4 family, this is underpinned also by the fact, that the sole I2SxExt signal (I2SxExt_SD, i.e. data) which is brought out to an external pin, is at the same pin but separate Alternative Function (AF) than the corresponding SPIx/I2Sx module's MISO pin (which is unused in I2S mode).

I2SxExt_SD is at a different AF in STM32F4xx

Maybe this has proven to be confusing to the users, because ST in the newer 'F3 family ST merged I2SxExt_SD with the corresponding SPIx_MISO to the same AF:

I2SxExt_SD STM32F3xx is at the same AF as corresponding SPIx_MISO

That I2SxExt is still a separate module, is witnessed by it occupying its own separate address space.