For creating a new control, you need to define the 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 __devinitdata = {
.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
};
Most likely the control is created via
snd_ctl_new1(), and in such a case, you can
add __devinitdata prefix to the
definition like above.
The iface field specifies the type of
the control, 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. On 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 subsection
Control Names.
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 detailed will be explained in the subsection
Access Flags.
The private_value field contains
an arbitrary long integer value for this record. When using
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 other three are callback functions.