To create a new control, you need to define the
following three
callbacks: info,
get and
put. Then, define a
struct snd_kcontrol_new record, such as:
Example 6.1. Definition of a Control
static struct snd_kcontrol_new my_control = {
.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
.name = "PCM Playback Switch",
.index = 0,
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
.private_value = 0xffff,
.info = my_control_info,
.get = my_control_get,
.put = my_control_put
};
The iface field specifies the control
type, SNDRV_CTL_ELEM_IFACE_XXX, which
is usually MIXER.
Use CARD for global controls that are not
logically part of the mixer.
If the control is closely associated with some specific device on
the sound card, use HWDEP,
PCM, RAWMIDI,
TIMER, or SEQUENCER, and
specify the device number with the
device and
subdevice fields.
The name is the name identifier
string. Since ALSA 0.9.x, the control name is very important,
because its role is classified from its name. There are
pre-defined standard control names. The details are described in
the
Control Names subsection.
The index field holds the index number
of this control. If there are several different controls with
the same name, they can be distinguished by the index
number. This is the case when
several codecs exist on the card. If the index is zero, you can
omit the definition above.
The access field contains the access
type of this control. Give the combination of bit masks,
SNDRV_CTL_ELEM_ACCESS_XXX, there.
The details will be explained in
the
Access Flags subsection.
The private_value field contains
an arbitrary long integer value for this record. When using
the generic info,
get and
put callbacks, you can pass a value
through this field. If several small numbers are necessary, you can
combine them in bitwise. Or, it's possible to give a pointer
(casted to unsigned long) of some record to this field, too.
The tlv field can be used to provide
metadata about the control; see the
Metadata subsection.
The other three are callback functions.