AUDIO(9) BSD Kernel Manual AUDIO(9)
audio - interface between low and high level audio drivers
The audio device driver is divided into a high level, hardware indepen-
dent 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 (*drain)(void *);
int (*query_encoding)(void *, struct audio_encoding *);
int (*set_params)(void *, int, int,
struct audio_params *, struct audio_params *);
int (*round_blocksize)(void *, int);
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 (*setfd)(void *, int);
int (*set_port)(void *, struct mixer_ctrl *);
int (*get_port)(void *, struct mixer_ctrl *);
int (*query_devinfo)(void *, struct mixer_devinfo *);
void *(*allocm)(void *, int, size_t, int, int);
void (*freem)(void *, void *, int);
size_t (*round_buffersize)(void *, int, size_t);
paddr_t (*mappage)(void *, void *, off_t, int);
int (*get_props)(void *);
int (*trigger_output)(void *, void *, void *, int,
void (*)(void *), void *, struct audio_params *);
int (*trigger_input)(void *, void *, void *, int,
void (*)(void *), void *, struct audio_params *);
};
struct audio_params {
u_long sample_rate; /* sample rate */
u_int encoding; /* mu-law, linear, etc */
u_int precision; /* bits/sample */
u_int channels; /* mono(1), stereo(2) */
/* Software en/decode functions, set if SW coding required by HW */
void (*sw_code)(void *, u_char *, int);
int factor; /* coding space change */
};
The high level audio driver attaches to the low level driver when the
latter calls audio_attach_mi(). This call is:
struct device *
audio_attach_mi(struct audio_hw_if *ahwp, void *hdl,
struct device *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 ahwp 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. During playback, if there is no data available
from the user process when the hardware requests another block, a block
of silence will be used instead. Similarly, if the user process does not
read data quickly enough during recording, data will be thrown away.
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)
This function is called when the audio device is opened, with
flags the kernel representation of flags passed to the open(2)
system call (see OFLAGS and FFLAGS in <sys/fcntl.h>). It initial-
izes the hardware for I/O. Every successful call to open() is
matched by a call to close(). This function returns 0 on success,
otherwise an error code.
void (*close)(void *hdl)
This function is called when the audio device is closed.
int (*drain)(void *hdl)
This function is optional. If supplied, it is called before the
device is closed or when the AUDIO_DRAIN ioctl(2) is called. It
makes sure that no samples remain to be played that could be lost
when close() is called. This function returns 0 on success, oth-
erwise an error code.
int (*query_encoding)(void *hdl, struct audio_encoding *ae)
This function is used when the AUDIO_GETENC ioctl(2) is called.
It fills ae and returns 0 or, if there is no encoding with the
given number, returns EINVAL.
int (*set_params)(void *hdl, int setmode, int usemode, struct
audio_params *play, struct audio_params *rec)
This function is called to set the audio encoding mode. setmode
is a combination of the AUMODE_RECORD and AUMODE_PLAY flags to
indicate which mode(s) are to be set. usemode is also a combina-
tion of these flags, but indicates the current mode of the device
(i.e., the value of mode in the audio_info struct). The play and
rec structures contain the encoding parameters that will be set.
If the hardware requires software assistance with some encoding
(e.g., it might be lacking mu-law support), it will fill the
sw_code and factor fields of these structures. See
/usr/src/sys/dev/auconv.h for available software support. The
values of the structures may also be modified if the hardware
cannot be set to exactly the requested mode (e.g., if the re-
quested sampling rate is not supported, but one close enough is).
If the device does not have the AUDIO_PROP_INDEPENDENT property,
the same value is passed in both play and rec and the encoding
parameters from play are copied into rec after the call to
set_params().
The machine independent audio driver does some preliminary param-
eter checking; it verifies that the precision is compatible with
the encoding, and it translates AUDIO_ENCODING_[US]LINEAR to
AUDIO_ENCODING_[US]LINEAR_{LE,BE}.
This function returns 0 on success, otherwise an error code.
int (*round_blocksize)(void *hdl, int bs)
This function is optional. If supplied, it is called with the
block size, bs, which has been computed by the upper layer. It
returns a block size, possibly changed according to the needs of
the hardware driver.
int (*commit_settings)(void *hdl)
This function is optional. If supplied, it is called after all
calls to set_params() 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. This function returns 0 on success, otherwise an
error code.
int (*init_output)(void *hdl, void *buffer, int size)
This function is optional. If supplied, it is called before any
output starts, but only after the total size of the output buffer
has been determined. It can be used to initialize looping DMA for
hardware that needs it. This function returns 0 on success, oth-
erwise an error code.
int (*init_input)(void *hdl, void *buffer, int size)
This function is optional. If supplied, it is called before any
input starts, but only after the total size of the input buffer
has been determined. It can be used to initialize looping DMA for
hardware that needs it. This function returns 0 on success, oth-
erwise an error code.
int (*start_output)(void *hdl, void *block, int bsize, void (*intr)(void
*), void *intrarg)
This function is called to start the transfer of bsize bytes from
block to the audio hardware. The call returns when the data
transfer has been initiated (normally with DMA). When the
hardware is ready to accept more samples the function intr will
be called with the argument intrarg. Calling intr will normally
initiate another call to start_output(). This function returns 0
on success, otherwise an error code.
int (*start_input)(void *hdl, void *block, int bsize, void (*intr)(void
*), void *intrarg)
This function is called to start the transfer of bsize bytes to
block from the audio hardware. The call returns when the data
transfer has been initiated (normally with DMA). When the
hardware is ready to deliver more samples the function intr will
be called with the argument intrarg. Calling intr will normally
initiate another call to start_input(). This function returns 0
on success, otherwise an error code.
int (*halt_output)(void *hdl)
This function is called to abort the output transfer (started by
start_output()) in progress. This function returns 0 on success,
otherwise an error code.
int (*halt_input)(void *hdl)
This function is called to abort the input transfer (started by
start_input()) in progress. This function returns 0 on success,
otherwise an error code.
int (*speaker_ctl)(void *hdl, int on)
This function is optional. If supplied, it is called when a half
duplex device changes between playing and recording. It can,
e.g., be used to turn the speaker on and off. This function re-
turns 0 on success, otherwise an error code.
int (*getdev)(void *hdl, struct audio_device *ret)
This function fills ret with relevant information about the
driver and returns 0 on success, or it returns an error code on
failure.
int (*setfd)(void *hdl, int fd)
This function is optional. If supplied, it is called when the
AUDIO_SETFD ioctl(2) is used, but only if the device has
AUDIO_PROP_FULLDUPLEX set. This function returns 0 on success,
otherwise an error code.
int (*set_port)(void *hdl, struct mixer_ctrl *mc)
This function is called when the AUDIO_MIXER_WRITE ioctl(2) is
used. It takes data from mc and sets the corresponding mixer
values. This function returns 0 on success, otherwise an error
code.
int (*get_port)(void *hdl, struct mixer_ctrl *mc)
This function is called when the AUDIO_MIXER_READ ioctl(2) is
used. It fills mc and returns 0 on success, or it returns an er-
ror code on failure.
int (*query_devinfo)(void *hdl, struct mixer_devinfo *di)
This function is called when the AUDIO_MIXER_DEVINFO ioctl(2) is
used. It fills di and returns 0 on success, or it returns an er-
ror code on failure.
void *(*allocm)(void *hdl, int direction, size_t size, int type, int
flags)
This function is optional. If supplied, it is called to allocate
the device buffers. If not supplied, 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. direction is AUMODE_PLAY
or AUMODE_RECORD. This function returns the address of the buffer
on success, or 0 on failure.
void (*freem)(void *hdl, void *addr, int type)
This function is optional. If supplied, it is called to free
memory allocated by allocm(). If not supplied, free(9) is used
instead.
size_t (*round_buffersize)(void *hdl, int direction, size_t bufsize)
This function is optional. If supplied, it 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. Note that the buffer size is always a multiple of the
block size, so round_blocksize() and round_buffersize() must be
consistent.
paddr_t (*mappage)(void *hdl, void *addr, off_t offs, int prot)
This function is optional. If supplied, it is called for mmap(2).
It returns the map value for the page at offset offs from address
addr mapped with protection prot. This function returns -1 on
failure, or a machine dependent opaque value on success.
int (*get_props)(void *hdl)
This function returns the device properties, as per audio(4)
AUDIO_GETPROPS ioctl(2), i.e., a combination of AUDIO_PROP_xxx
properties.
int (*trigger_output)(void *hdl, void *start, void *end, int blksize,
void (*intr)(void *), void *intrarg, struct audio_params *param)
This function is optional. If supplied, it 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
returns when the data transfer has been initiated (normally with
DMA). When the hardware is finished transferring each blksize
sized block, the function intr will be called with the argument
intrarg (typically from the audio hardware interrupt service rou-
tine). Once started, the transfer may be stopped using
halt_output(). This function returns 0 on success, otherwise an
error code.
int (*trigger_input)(void *hdl, void *start, void *end, int blksize, void
(*intr)(void *), void *intrarg, struct audio_params *param)
This function is optional. If supplied, it 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 returns when the data transfer has been initiated (normally
with DMA). When the hardware is finished transferring each
blksize sized block, the function intr will be called with the
argument intrarg (typically from the audio hardware interrupt
service routine). Once started, the transfer may be stopped using
halt_input(). This function returns 0 on success, otherwise an
error code.
The query_devinfo() method should define certain mixer controls for
AUDIO_SETINFO to be able to change the port and gain.
If the audio hardware 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. For each of the named sources there should be a
control in the AudioCinputs class of type AUDIO_MIXER_VALUE if recording
level of the source can be set. If the overall recording level can be
changed (i.e., regardless of the input source) then this control should
be named AudioNrecord and be of class AudioCinputs.
If the audio hardware is capable of output to more than one destination
it should define AudioNoutput in class AudioCmonitor. 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 there should be
a control in the AudioCoutputs class of type AUDIO_MIXER_VALUE if output
level of the destination can be set. If the overall output level can be
changed (i.e., regardless of the destination) then this control should be
named AudioNmaster and be of class AudioCoutputs.
ioctl(2), mmap(2), open(2), audio(4), free(9), malloc(9)
This audio interface first appeared in OpenBSD 1.2.
MirBSD #10-current February 11, 2000 4