Chapter 10. Miscellaneous Devices
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# Chapter 10. Miscellaneous Devices

## FM OPL3

The FM OPL3 is still used in many chips (mainly for backward compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API is defined in `<sound/opl3.h>`.

FM registers can be directly accessed through the direct-FM API, defined in `<sound/asound_fm.h>`. In ALSA native mode, FM registers are accessed through the Hardware-Dependent Device direct-FM extension API, whereas in OSS compatible mode, FM registers can be accessed with the OSS direct-FM compatible API in `/dev/dmfmX` device.

To create the OPL3 component, you have two functions to call. The first one is a constructor for the opl3_t instance.

      struct snd_opl3 *opl3;
      snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
                      integrated, &opl3);

The first argument is the card pointer, the second one is the left port address, and the third is the right port address. In most cases, the right port is placed at the left port + 2.

The fourth argument is the hardware type.

When the left and right ports have been already allocated by the card driver, pass non-zero to the fifth argument (`integrated`). Otherwise, the opl3 module will allocate the specified ports by itself.

When the accessing the hardware requires special method instead of the standard I/O access, you can create opl3 instance separately with `snd_opl3_new()`.

      struct snd_opl3 *opl3;
      snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);

Then set `command`, `private_data` and `private_free` for the private access function, the private data and the destructor. The l_port and r_port are not necessarily set. Only the command must be set properly. You can retrieve the data from the opl3->private_data field.

After creating the opl3 instance via `snd_opl3_new()`, call `snd_opl3_init()` to initialize the chip to the proper state. Note that `snd_opl3_create()` always calls it internally.

If the opl3 instance is created successfully, then create a hwdep device for this opl3.

      struct snd_hwdep *opl3hwdep;
      snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);

The first argument is the opl3_t instance you created, and the second is the index number, usually 0.

The third argument is the index-offset for the sequencer client assigned to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART always takes 0).

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