AUDIO(9) | Kernel Developer's Manual | AUDIO(9)

# NAME

`audio` — interface between low and high level audio drivers

# DESCRIPTION

The audio device driver is divided into a high level, hardware independent layer, and a low level hardware dependent layer. The interface between these is the audio_hw_if structure.

    struct audio_hw_if {
    	int	(*open)(void *, int);
    	void	(*close)(void *);

    	int	(*query_format)(void *, audio_format_query_t *);
    	int	(*set_format)(void *, int,
    	            const audio_params_t *, const audio_params_t *,
    	            audio_filter_reg_t *, audio_filter_reg_t *);
    	int	(*round_blocksize)(void *, int, int, const audio_params_t *);

    	int	(*commit_settings)(void *);

    	int	(*init_output)(void *, void *, int);
    	int	(*init_input)(void *, void *, int);
    	int	(*start_output)(void *, void *, int, void (*)(void *),
    	            void *);
    	int	(*start_input)(void *, void *, int, void (*)(void *),
    		    void *);
    	int	(*halt_output)(void *);
    	int	(*halt_input)(void *);

    	int	(*speaker_ctl)(void *, int);
    #define SPKR_ON  1
    #define SPKR_OFF 0

    	int	(*getdev)(void *, struct audio_device *);

    	int	(*set_port)(void *, mixer_ctrl_t *);
    	int	(*get_port)(void *, mixer_ctrl_t *);

    	int	(*query_devinfo)(void *, mixer_devinfo_t *);

    	void	*(*allocm)(void *, int, size_t);
    	void	(*freem)(void *, void *, size_t);
    	size_t	(*round_buffersize)(void *, int, size_t);

    	int 	(*get_props)(void *);

    	int	(*trigger_output)(void *, void *, void *, int,
    		    void (*)(void *), void *, const audio_params_t *);
    	int	(*trigger_input)(void *, void *, void *, int,
    		    void (*)(void *), void *, const audio_params_t *);
    	int	(*dev_ioctl)(void *, u_long, void *, int, struct lwp *);
    	void	(*get_locks)(void *, kmutex_t **, kmutex_t **);
    };

    typedef struct audio_params {
    	u_int	sample_rate;	/* sample rate */
    	u_int	encoding;	/* e.g. mu-law, linear, etc */
    	u_int	precision;	/* bits/subframe */
    	u_int	validbits;	/* valid bits in a subframe */
    	u_int	channels;	/* mono(1), stereo(2) */
    } audio_params_t;

The high level audio driver attaches to the low level driver when the latter calls audio_attach_mi. This call should be

        device_t
        audio_attach_mi(const struct audio_hw_if *ahwp, void *hdl, device_t dev);

The audio_hw_if struct is as shown above. The hdl argument is a handle to some low level data structure. It is sent as the first argument to all the functions in audio_hw_if when the high level driver calls them. dev is the device struct for the hardware device.

The upper layer of the audio driver allocates one buffer for playing and one for recording. It handles the buffering of data from the user processes in these. The data is presented to the lower level in smaller chunks, called blocks. If, during playback, there is no data available from the user process when the hardware request another block a block of silence will be used instead. Furthermore, if the user process does not read data quickly enough during recording data will be thrown away.

The phase that these functions are called is classified into three. Attach phase, Closed phase and Opened phase. Attach phase is during device attach and it transits to the Closed phase when the attach succeeded. Closed phase is when no sampling device is opened and it transits to the Opened phase when open succeeded. Opened phase is when any sampling device is opened and it transits to the Closed phase when close succeeded.

The fields of audio_hw_if are described in some more detail below. Some fields are optional and can be set to `NULL` if not needed.

`int open(void *hdl, int flags)`
  optional, is called when the first device combining playback and recording is opened. On a full duplex hardware, (`FREAD` | `FWRITE`) is passed to flags. On a half duplex hardware, `FWRITE` is passed for playback, or `FREAD` for recording. Every successful call to open is matched by a call to close. Return 0 on success, otherwise an error code. It is called in the Closed phase.
`void close(void *hdl)`
  optional, is called when the last audio device combining playback and recording is closed. Before call to this, halt_input and halt_output are called if necessary. It is called in the Opened phase.
`int query_format(void *hdl, audio_format_query_t *afp)`
  is called to enumerate formats supported by the hardware. It should fill the audio_format_t structure according to given number afp->index. If there is no format with the given number, return `EINVAL`. It can be called at any time.

      typedef struct audio_format_query {
      	u_int	index;
      	struct audio_format fmt;
      } audio_format_query_t;

  It is also used by the upper layer to determine the default format, as follows:

  1. Higher priority is preferred (normally 0, the highest is 3, the lowest is 0).
  2. `AUDIO_ENCODING_SLINEAR_NE:16` is preferred if exists.
  3. `AUDIO_ENCODING_SLINEAR_OE:16` is preferred if exists.
  4. The format with more channels is preferred.

  If the driver supports `SLINEAR_NE:16` and the upper layer chooses it, the driver does not need to provide a conversion function in set_format. Similarly, if the driver supports `SLINEAR_OE:16` and the upper layer chooses it, the driver does not need to provide a conversion function, because the upper layer supports conversion between `SLINEAR_NE:16` and `SLINEAR_OE:16` for convenience. If the upper layer chooses another format, the driver needs to provide a conversion function in set_format. See also set_format. If the driver can not provide the conversion from/to `SLINEAR_NE:16`, set priority to -1. It means that the hardware supports this format but the driver does not (e.g. AC3), and it will never be chosen.

`int set_format(void *hdl, int setmode,`
  `const audio_params_t *play, const audio_params_t *rec,` `audio_filter_reg_t *pfil, audio_filter_reg_t *rfil)`

  is called to set specified format to the hardware, when the device is attached or the hardware format is changed. setmode is a combination of the `AUMODE_RECORD` and `AUMODE_PLAY` flags to indicate which modes are to be set.

  The play and rec structures contain the encoding parameters that should be set to the hardware. All of these parameters are chosen from formats returned by query_format. Therefore play and/or rec are always settable. If the hardware does not support `AUDIO_ENCODING_SLINEAR_{NE,OE}:16`, conversion information should be filled the pfil for playing or rfil for recording. The definition of audio_filter_reg_t and a related structure follow:

      typedef struct {
      	const void *src;
      	const audio_format2_t *srcfmt;
      	void *dst;
      	const audio_format2_t *dstfmt;
      	int count;
      	void *context;
      } audio_filter_arg_t;

      typedef void(*audio_filter_t)(audio_filter_arg_t *arg);

      typedef struct {
      	audio_filter_t codec;
      	void *context;
      } audio_filter_reg_t;

  codec is a conversion function and context is an optional opaque pointer passed to codec.

  When codec is called, all parameters required by codec are contained in arg. src points to the input buffer block, srcfmt contains the input encoding parameters, dst points to the output buffer block and dstfmt contains the output encoding parameters. count represents the number of frames to process on this call. src and dst are guaranteed to be able to consecutively access number of frames specified by count. codec must fill the entirety of dst. For example, let count = 100, srcfmt is { precision = 16, channels = 3 }, dstfmt is { precision = 8, channels = 4 }, in this case, src block length = 2(bytes) * 3(channels) * 100(frames) = 600 bytes, The length to be written to dst block is 1(byte) * 4(channels) * 100(frames) = 400 bytes. codec cannot abort the conversion halfway and there is no error reporting mechanism. context is a opaque pointer that can be used by codec if necessary.

  If the device does not have the `AUDIO_PROP_INDEPENDENT` property the same value is passed in both play and rec. Returns 0 on success, otherwise an error code. It is called in the Attach or Closed phases.

`int round_blocksize(void *hdl, int bs, int mode,`
  `const audio_params_t *param)`

  optional, is called with the block size, bs, that has been computed by the upper layer, mode, `AUMODE_PLAY` or `AUMODE_RECORD`, and param, encoding parameters for the hardware. bs passed is always non-zero and a multiple of the frame size represented by param->channels * param->precision / 8. It should return a block size, possibly changed according to the needs of the hardware driver. The return value also must be non-zero and a multiple of the frame size. It is called in the Attach or Closed phases.

`int commit_settings(void *hdl)`
  optional, is called after all calls to set_format, and set_port, are done. A hardware driver that needs to get the hardware in and out of command mode for each change can save all the changes during previous calls and do them all here. Returns 0 on success, otherwise an error code. It is called in the Attach or Closed phases.
`int init_output(void *hdl, void *buffer, int size)`
  optional, is called before any output starts, but when the total size of the output buffer has been determined. It can be used to initialize looping DMA for hardware that needs that. Return 0 on success, otherwise an error code. It is called in the Attach or Closed phases.
`int init_input(void *hdl, void *buffer, int size)`
  optional, is called before any input starts, but when the total size of the input buffer has been determined. It can be used to initialize looping DMA for hardware that needs that. Returns 0 on success, otherwise an error code. It is called in the Attach or Closed phases.
`int start_output(void *hdl, void *block, int blksize,`
  `void (*intr)(void*), void *intrarg)`

  is called to start the transfer of blksize bytes from block to the audio hardware. The call should return when the data transfer has been initiated (normally with DMA). When the hardware is ready to accept more samples the function intr should be called with the argument intrarg. Calling intr will normally initiate another call to start_output. Returns 0 on success, otherwise an error code. This field is optional only if the driver doesn't support playback. It is called in the Opened phase.

`int start_input(void *hdl, void *block, int blksize,`
  `void (*intr)(void*), void *intrarg)`

  is called to start the transfer of blksize bytes to block from the audio hardware. The call should return when the data transfer has been initiated (normally with DMA). When the hardware is ready to deliver more samples the function intr should be called with the argument intrarg. Calling intr will normally initiate another call to start_input. Returns 0 on success, otherwise an error code. This field is optional only if the driver doesn't support recording. It is called in the Opened phase.

`int halt_output(void *hdl)`
  is called to abort the output transfer (started by start_output) in progress. Returns 0 on success, otherwise an error code. This field is optional only if the driver doesn't support playback. It is called in the Opened phase.
`int halt_input(void *hdl)`
  is called to abort the input transfer (started by start_input) in progress. Returns 0 on success, otherwise an error code. This field is optional only if the driver doesn't support recording, It is called in the Opened phase.
`int speaker_ctl(void *hdl, int on)`
  optional, is called when a half duplex device changes between playing and recording. It can, e.g., be used to turn on and off the speaker. Returns 0 on success, otherwise an error code. It is called in the Opened phase.
`int getdev(void *hdl, struct audio_device *ret)`
  Should fill the audio_device struct with relevant information about the driver. Returns 0 on success, otherwise an error code. It is called in the Opened phase.
`int set_port(void *hdl, mixer_ctrl_t *mc)`
  is called in when `AUDIO_MIXER_WRITE` is used. It should take data from the mixer_ctrl_t struct and set the corresponding mixer values. Returns 0 on success, otherwise an error code. It is called in the Opened or Closed phases.
`int get_port(void *hdl, mixer_ctrl_t *mc)`
  is called in when `AUDIO_MIXER_READ` is used. It should fill the mixer_ctrl_t struct. Returns 0 on success, otherwise an error code. It is called in the Opened or Closed phases.
`int query_devinfo(void *hdl, mixer_devinfo_t *di)`
  is called in when `AUDIO_MIXER_DEVINFO` is used. It should fill the mixer_devinfo_t struct. Return 0 on success, otherwise an error code. It is called at any time.
`void *allocm(void *hdl, int direction, size_t size)`
  optional, is called to allocate the device buffers. If not present malloc(9) is used instead (with the same arguments but the first two). The reason for using a device dependent routine instead of malloc(9) is that some buses need special allocation to do DMA. Returns the address of the buffer, or `NULL` on failure. It is called in the Attached or Closed phases.
`void freem(void *hdl, void *addr, size_t size)`
  optional, is called to free memory allocated by allocm. If not supplied free(9) is used. It is called in the Attached or Closed phases.
`size_t round_buffersize(void *hdl, int direction, size_t bufsize)`
  optional, is called at startup to determine the audio buffer size. The upper layer supplies the suggested size in bufsize, which the hardware driver can then change if needed. E.g., DMA on the ISA bus cannot exceed 65536 bytes. It is called in the Attached or Closed phases.
`int get_props(void *hdl)`
  Should return the device properties in a combination of following flags:

  `AUDIO_PROP_PLAYBACK`
    the device is capable of audio playback.
  `AUDIO_PROP_CAPTURE`
    the device is capable of audio capture.
  `AUDIO_PROP_FULLDUPLEX`
    the device admits full duplex operation. Don't set it if the device is unidirectional.
  `AUDIO_PROP_INDEPENDENT`
    the device can set the playing and recording encoding parameters independently. Don't set it if the device is unidirectional.
  `AUDIO_PROP_MMAP`
    is handled in the upper layer, so new drivers should not return this property.
  It is called in the Attach phase.
`int trigger_output(void *hdl, void *start, void *end,`
  `int blksize, void (*intr)(void*), void *intrarg,`

  `const audio_params_t *param)`

  optional, is called to start the transfer of data from the circular buffer delimited by start and end to the audio hardware, parameterized as in param. The call should return when the data transfer has been initiated (normally with DMA). When the hardware is finished transferring each blksize sized block, the function intr should be called with the argument intrarg (typically from the audio hardware interrupt service routine). Once started the transfer may be stopped using halt_output. Return 0 on success, otherwise an error code. It is called in the Opened phase.

`int trigger_input(void *hdl, void *start, void *end,`
  `int blksize, void (*intr)(void*), void *intrarg,`

  `const audio_params_t *param)`

  optional, is called to start the transfer of data from the audio hardware, parameterized as in param, to the circular buffer delimited by start and end. The call should return when the data transfer has been initiated (normally with DMA). When the hardware is finished transferring each blksize sized block, the function intr should be called with the argument intrarg (typically from the audio hardware interrupt service routine). Once started the transfer may be stopped using halt_input. Return 0 on success, otherwise an error code. It is called in the Opened phase.

`int dev_ioctl(void *hdl, u_long cmd, void *addr,`

  `int flag, struct lwp *l)`

  optional, is called when an ioctl(2) is not recognized by the generic audio driver. Return 0 on success, otherwise an error code. It is called in the Opened phase.

`void get_locks(void *hdl, kmutex_t **intr, kmutex_t **thread)`
  Returns the interrupt and thread locks to the common audio layer. It is called in the Attach phase.

The query_devinfo method should define certain mixer controls for `AUDIO_SETINFO` to be able to change the port and gain, and `AUDIO_GETINFO` to read them, as follows.

If the record mixer is capable of input from more than one source, it should define `AudioNsource` in class `AudioCrecord`. This mixer control should be of type `AUDIO_MIXER_ENUM` or `AUDIO_MIXER_SET` and enumerate the possible input sources. Each of the named sources for which the recording level can be set should have a control in the `AudioCrecord` class of type `AUDIO_MIXER_VALUE`, except the "mixerout" source is special, and will never have its own control. Its selection signifies, rather, that various sources in class `AudioCrecord` will be combined and presented to the single recording output in the same fashion that the sources of class `AudioCinputs` are combined and presented to the playback output(s). If the overall recording level can be changed, regardless of the input source, then this control should be named `AudioNmaster` and be of class `AudioCrecord`.

Controls for various sources that affect only the playback output, as opposed to recording, should be in the `AudioCinputs` class, as of course should any controls that affect both playback and recording.

If the play mixer is capable of output to more than one destination, it should define `AudioNselect` in class `AudioCoutputs`. This mixer control should be of type `AUDIO_MIXER_ENUM` or `AUDIO_MIXER_SET` and enumerate the possible destinations. For each of the named destinations for which the output level can be set, there should be a control in the `AudioCoutputs` class of type `AUDIO_MIXER_VALUE`. If the overall output level can be changed, which is invariably the case, then this control should be named `AudioNmaster` and be of class `AudioCoutputs`.

There's one additional source recognized specially by `AUDIO_SETINFO` and `AUDIO_GETINFO`, to be presented as monitor_gain, and that is a control named `AudioNmonitor`, of class `AudioCmonitor`.

# SEE ALSO

audio(4)

# HISTORY

This `audio` interface first appeared in NetBSD 1.3.

February 2, 2021 | NetBSD 11.0