Control接口主要让用户空间的应用程序（alsa-lib）可以访问和控制音频codec芯片中的多路开关，滑动控件等。对于 Mixer （混音）来说，Control接口显得尤为重要，从ALSA 0.9.x版本开始，所有的mixer工作都是通过control接口的API来实现的。
ALSA已经为AC97定义了完整的控制接口模型，如果你的Codec芯片只支持AC97接口，你可以不用关心本节的内容。
<sound/control.h>定义了所有的Control API。如果你要为你的codec实现自己的controls，请在代码中包含该头文件。
1、snd_kcontrol_new
struct snd_kcontrol_new {
snd_ctl_elem_iface_t iface; /* interface identifier */
unsigned int device; /* device/client number */
unsigned int subdevice; /* subdevice (substream) number */
const char *name; /* ASCII name of item */
unsigned int index; /* index of item */
unsigned int access; /* access rights */
unsigned int count; /* count of same elements */
snd_kcontrol_info_t *info;
snd_kcontrol_get_t *get;
snd_kcontrol_put_t *put;
union {
snd_kcontrol_tlv_rw_t *c;
const unsigned int *p;
} tlv;
unsigned long private_value;
};
iface：表示control的类型，用SNDRV_CTL_ELEM_IFACE_XXX来定义。通常使用MIXER，也可以定于属于全局的CARD类型，如果定义为属于莫雷设备的类型，例如HWDEP、PCMRAWMIDI、TIMER等，此时必须在device和subdevice字段中支出卡的设备逻辑编号。
name：表示control的名字，用户层可以通过这个名字访问这个control，后续会细聊
index：存放这个 control 的索引号。如果声卡下不止一个codec。每个codec有相同的名字的control。此时就需要通过index来区分这些controls，当index为0，则可以忽略这种区分策略。
access：访问权限的控制，READ，WRITE，READWRITE等。每一个bit代表一种访问类型，这些访问类型可以多个或运算组合在一起使用。
private_value：包含了一个人员的长整数类型的值，该值可以通过info、get、put这几个回调函数访问。
tlv：该字段为control提供元数据。
2、control的名字
control的名字需要遵循一些标准，通常可以分成3部分来定义control的名字：源--方向--功能。
源：可以理解为该control的输入端，alsa已经预定义了一些常用的源，例如：Master，PCM，CD，Line等等。
方向：代表该control的数据流向，例如：Playback，Capture，Bypass，Bypass Capture等等，也可以不定义方向，这时表示该Control是双向的（playback和capture）。
功能：根据control的功能，可以是以下字符串：Switch，Volume，Route等等。
也有一些命名上的特例：
1、全局的capture和playback："Capture Source"，“Capture Volume”，“Capture Switch”，他们用于全局的capture source、switch和volume。同样的“Playback Volume”，“Playeback Switch”，它们用于全局的输出switch和volume。
2、Tone-controles：音调控制的开关和音量命名为：Tomw Control-XXX，例如，“Tone-Control-Switch”，“Tone Control-Bass”，“Tone Control-Center”。
3、3D controls：3D控件的命名规则：“3D Control-Switch”,“3D Control-Center”,“3D Control-Space”。
4、MIC boost：麦克风音量加强空间命名为：“MIC Boost”或“MIC Bosst（6dB）”。
3、访问标志（ACCESS Flags）
Access字段是一个bitmask，它保存了改control的访问类型。默认的访问类型是：SNDDRV_CTL_ELEM_ACCESS_READWRITE，表明该control支持读和写操作。如果access字段没有定义（.access==0），此时也认为是READWRITE类型。
如果是一个只读control，access应该设置为：SNDDRV_CTL_ELEM_ACCESS_READ，这时，我们不必定义put回调函数。类似地，如果是只写control，access应该设置为：SNDDRV_CTL_ELEM_ACCESS_WRITE，这时，我们不必定义get回调函数。
如果control的值会频繁地改变（例如：电平表），我们可以使用VOLATILE类型，这意味着该control会在没有通知的情况下改变，应用程序应该定时地查询该control的值。
4、元数据（METADATA）
很多mixer control需要提供以dB为单位的信息，我们可以使用DECLARE_TLV_xxx宏来定义一些包含这种信息的变量，然后把control的tlv.p字段指向这些变量，最后，在access字段中加上SNDRV_CTL_ELEM_ACCESS_TLV_READ标志，例如：
static const DECLARE_TLV_DB_SCALE(snd_cx88_db_scale, -6300, 100, 0);
static const struct snd_kcontrol_new snd_cx88_volume = {
.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
SNDRV_CTL_ELEM_ACCESS_TLV_READ,
.name = "Analog-TV Volume",
.info = snd_cx88_volume_info,
.get = snd_cx88_volume_get,
.put = snd_cx88_volume_put,
.tlv.p = snd_cx88_db_scale,
};
5、函数详解
5.1、snd_ctl_new1函数
/**
* snd_ctl_new1 - create a control instance from the template
* @ncontrol: the initialization record
* @private_data: the private data to set
*
* Allocates a new struct snd_kcontrol instance and initialize from the given
* template. When the access field of ncontrol is 0, it's assumed as
* READWRITE access. When the count field is 0, it's assumes as one.
*
* Return: The pointer of the newly generated instance, or %NULL on failure.
*/
struct snd_kcontrol *snd_ctl_new1(const struct snd_kcontrol_new *ncontrol,
void *private_data)
{
struct snd_kcontrol *kctl;
unsigned int count;
unsigned int access;
int err;
if (snd_BUG_ON(!ncontrol || !ncontrol->info))
return NULL;
count = ncontrol->count;
if (count == 0)
count = 1;
access = ncontrol->access;
if (access == 0)
access = SNDRV_CTL_ELEM_ACCESS_READWRITE;
access &= (SNDRV_CTL_ELEM_ACCESS_READWRITE |
SNDRV_CTL_ELEM_ACCESS_VOLATILE |
SNDRV_CTL_ELEM_ACCESS_INACTIVE |
SNDRV_CTL_ELEM_ACCESS_TLV_READWRITE |
SNDRV_CTL_ELEM_ACCESS_TLV_COMMAND |
SNDRV_CTL_ELEM_ACCESS_TLV_CALLBACK |
SNDRV_CTL_ELEM_ACCESS_LED_MASK |
SNDRV_CTL_ELEM_ACCESS_SKIP_CHECK);
/* 创建snd_kcontrol */
err = snd_ctl_new(&kctl, count, access, NULL);
if (err < 0)
return NULL;
/* 根据snd_kcontrol_new初始化snd_kcontrol */
/* The 'numid' member is decided when calling snd_ctl_add(). */
kctl->id.iface = ncontrol->iface;
kctl->id.device = ncontrol->device;
kctl->id.subdevice = ncontrol->subdevice;
if (ncontrol->name) {
strscpy(kctl->id.name, ncontrol->name, sizeof(kctl->id.name));
if (strcmp(ncontrol->name, kctl->id.name) != 0)
pr_warn("ALSA: Control name '%s' truncated to '%s'\n",
ncontrol->name, kctl->id.name);
}
kctl->id.index = ncontrol->index;
kctl->info = ncontrol->info;
kctl->get = ncontrol->get;
kctl->put = ncontrol->put;
kctl->tlv.p = ncontrol->tlv.p;
kctl->private_value = ncontrol->private_value;
kctl->private_data = private_data;
return kctl;
}
分配一个新的snd_kcontrol实例,并把my_control中相应的值复制到该实例中,所以,在定义my_control时,通常我们可以加上__devinitdata前缀.snd_ctl_add则把该control绑定到声卡对象card当中。
struct snd_kcontrol {
struct list_head list; /* list of controls */
struct snd_ctl_elem_id id;
unsigned int count; /* count of same elements */
snd_kcontrol_info_t *info;
snd_kcontrol_get_t *get;
snd_kcontrol_put_t *put;
union {
snd_kcontrol_tlv_rw_t *c;
const unsigned int *p;
} tlv;
unsigned long private_value;
void *private_data;
void (*private_free)(struct snd_kcontrol *kcontrol);
struct snd_kcontrol_volatile vd[]; /* volatile data */
};
#define snd_kcontrol(n) list_entry(n, struct snd_kcontrol, list)
5.2、 snd_ctl_add函数
/* add/replace a new kcontrol object; call with card->controls_rwsem locked */
static int __snd_ctl_add_replace(struct snd_card *card,
struct snd_kcontrol *kcontrol,
enum snd_ctl_add_mode mode)
{
struct snd_ctl_elem_id id;
unsigned int idx;
struct snd_kcontrol *old;
int err;
id = kcontrol->id;
if (id.index > UINT_MAX - kcontrol->count)
return -EINVAL;
old = snd_ctl_find_id(card, &id);
if (!old) {
if (mode == CTL_REPLACE)
return -EINVAL;
} else {
if (mode == CTL_ADD_EXCLUSIVE) {
dev_err(card->dev,
"control %i:%i:%i:%s:%i is already present\n",
id.iface, id.device, id.subdevice, id.name,
id.index);
return -EBUSY;
}
err = snd_ctl_remove(card, old);
if (err < 0)
return err;
}
if (snd_ctl_find_hole(card, kcontrol->count) < 0)
return -ENOMEM;
/* 把snd_kcontrol挂入snd_card的controls链表 */
list_add_tail(&kcontrol->list, &card->controls);
card->controls_count += kcontrol->count;
/* 设置元素ID */
kcontrol->id.numid = card->last_numid + 1;
card->last_numid += kcontrol->count;
for (idx = 0; idx < kcontrol->count; idx++)
snd_ctl_notify_one(card, SNDRV_CTL_EVENT_MASK_ADD, kcontrol, idx);
return 0;
}
static int snd_ctl_add_replace(struct snd_card *card,
struct snd_kcontrol *kcontrol,
enum snd_ctl_add_mode mode)
{
int err = -EINVAL;
if (! kcontrol)
return err;
if (snd_BUG_ON(!card || !kcontrol->info))
goto error;
down_write(&card->controls_rwsem);
err = __snd_ctl_add_replace(card, kcontrol, mode);
up_write(&card->controls_rwsem);
if (err < 0)
goto error;
return 0;
error:
snd_ctl_free_one(kcontrol);
return err;
}
/**
* snd_ctl_add - add the control instance to the card
* @card: the card instance
* @kcontrol: the control instance to add
*
* Adds the control instance created via snd_ctl_new() or
* snd_ctl_new1() to the given card. Assigns also an unique
* numid used for fast search.
*
* It frees automatically the control which cannot be added.
*
* Return: Zero if successful, or a negative error code on failure.
*
*/
int snd_ctl_add(struct snd_card *card, struct snd_kcontrol *kcontrol)
{
return snd_ctl_add_replace(card, kcontrol, CTL_ADD_EXCLUSIVE);
}
5.3、info回调函数
用于得到对应control的详细信息，需要把信息存入snd_ctl_elem_info 对象中。
struct snd_ctl_elem_info {
struct snd_ctl_elem_id id; /* W: element ID */
snd_ctl_elem_type_t type; /* R: value type - SNDRV_CTL_ELEM_TYPE_* */
unsigned int access; /* R: value access (bitmask) - SNDRV_CTL_ELEM_ACCESS_* */
unsigned int count; /* count of values */
__kernel_pid_t owner; /* owner's PID of this control */
union {
struct {
long min; /* R: minimum value */
long max; /* R: maximum value */
long step; /* R: step (0 variable) */
} integer;
struct {
long long min; /* R: minimum value */
long long max; /* R: maximum value */
long long step; /* R: step (0 variable) */
} integer64;
struct {
unsigned int items; /* R: number of items */
unsigned int item; /* W: item number */
char name[64]; /* R: value name */
__u64 names_ptr; /* W: names list (ELEM_ADD only) */
unsigned int names_length;
} enumerated;
unsigned char reserved[128];
} value;
unsigned char reserved[64];
};
其中的value是个一个共用体，需要根据control的类型，确定值的类型，control type包括如下几类：
typedef int __bitwise snd_ctl_elem_type_t;
#define SNDRV_CTL_ELEM_TYPE_NONE ((__force snd_ctl_elem_type_t) 0) /* invalid */
#define SNDRV_CTL_ELEM_TYPE_BOOLEAN ((__force snd_ctl_elem_type_t) 1) /* boolean type */
#define SNDRV_CTL_ELEM_TYPE_INTEGER ((__force snd_ctl_elem_type_t) 2) /* integer type */
#define SNDRV_CTL_ELEM_TYPE_ENUMERATED ((__force snd_ctl_elem_type_t) 3) /* enumerated type */
#define SNDRV_CTL_ELEM_TYPE_BYTES ((__force snd_ctl_elem_type_t) 4) /* byte array */
#define SNDRV_CTL_ELEM_TYPE_IEC958 ((__force snd_ctl_elem_type_t) 5) /* IEC958 (S/PDIF) setup */
#define SNDRV_CTL_ELEM_TYPE_INTEGER64 ((__force snd_ctl_elem_type_t) 6) /* 64-bit integer type */
#define SNDRV_CTL_ELEM_TYPE_LAST SNDRV_CTL_ELEM_TYPE_INTEGER64
下面是以SNDRV_CTL_ELEM_TYPE_INTEGER和以SNDRV_CTL_ELEM_TYPE_BOOLEAN为例定义的info回调函数：
static int snd_cx88_volume_info(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_info *info)
{
info->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
info->count = 2;
info->value.integer.min = 0;
info->value.integer.max = 0x3f;
return 0;
}
static int snd_saa7134_capsrc_info(struct snd_kcontrol * kcontrol,
struct snd_ctl_elem_info * uinfo)
{
uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
uinfo->count = 2;
uinfo->value.integer.min = 0;
uinfo->value.integer.max = 1;
return 0;
}
5.4、get回调函数
这个函数用来读取当前 control 的值并返回到用户空间，需要把值放在snd_ctl_elem_value结构体中，与info结构体类似，value字段是一个共用体，与类型相关。如果value的cont大于1, 需要把值全部放入到 value[]数组中。
static int snd_cx88_volume_info(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_info *info)
{
info->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
info->count = 2;
info->value.integer.min = 0;
info->value.integer.max = 0x3f;
return 0;
}
static int snd_cx88_volume_get(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *value)
{
struct cx88_audio_dev *chip = snd_kcontrol_chip(kcontrol);
struct cx88_core *core = chip->core;
int vol = 0x3f - (cx_read(AUD_VOL_CTL) & 0x3f),
bal = cx_read(AUD_BAL_CTL);
value->value.integer.value[(bal & 0x40) ? 0 : 1] = vol;
vol -= (bal & 0x3f);
value->value.integer.value[(bal & 0x40) ? 1 : 0] = vol < 0 ? 0 : vol;
return 0;
}
5.5、put回调函数
put回调函数用于把应用程序的控制值设置到control中。
static void snd_cx88_wm8775_volume_put(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *value)
{
struct cx88_audio_dev *chip = snd_kcontrol_chip(kcontrol);
struct cx88_core *core = chip->core;
u16 left = value->value.integer.value[0];
u16 right = value->value.integer.value[1];
int v, b;
/* Pass volume & balance onto any WM8775 */
if (left >= right) {
v = left << 10;
b = left ? (0x8000 * right) / left : 0x8000;
} else {
v = right << 10;
b = right ? 0xffff - (0x8000 * left) / right : 0x8000;
}
wm8775_s_ctrl(core, V4L2_CID_AUDIO_VOLUME, v);
wm8775_s_ctrl(core, V4L2_CID_AUDIO_BALANCE, b);
}
/* OK - TODO: test it */
static int snd_cx88_volume_put(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *value)
{
struct cx88_audio_dev *chip = snd_kcontrol_chip(kcontrol);
struct cx88_core *core = chip->core;
int left, right, v, b;
int changed = 0;
u32 old;
if (core->sd_wm8775)
snd_cx88_wm8775_volume_put(kcontrol, value);
left = value->value.integer.value[0] & 0x3f;
right = value->value.integer.value[1] & 0x3f;
b = right - left;
if (b < 0) {
v = 0x3f - left;
b = (-b) | 0x40;
} else {
v = 0x3f - right;
}
/* Do we really know this will always be called with IRQs on? */
spin_lock_irq(&chip->reg_lock);
old = cx_read(AUD_VOL_CTL);
if (v != (old & 0x3f)) {
cx_swrite(SHADOW_AUD_VOL_CTL, AUD_VOL_CTL, (old & ~0x3f) | v);
changed = 1;
}
if ((cx_read(AUD_BAL_CTL) & 0x7f) != b) {
cx_write(AUD_BAL_CTL, b);
changed = 1;
}
spin_unlock_irq(&chip->reg_lock);
return changed;
}
6、Control设备创建流程
Control设备和PCM设备一样，都属于声卡下的逻辑设备。用户空间的应用程序通过alsa-lib访问该Control设备，读取或设置control的控制状态，从而达到控制音频Codec进行各种Mixer等控制操作。
Control设备的创建过程大体上和PCM设备的创建过程相同。详细的创建过程可以参考下方时序图。
我们需要在我们的驱动程序初始化时主动调用snd_pcm_new()函数创建pcm设备，而control设备则在snd_ctl_new1()内被创建，snd_ctl_new1()通过调用snd_ctl_create()函数创建control设备节点。所以我们无需显式地创建control设备，只要建立声卡，control设备被自动地创建。
和pcm设备一样，control设备的名字遵循一定的规则：controlCxx，这里的xx代表声卡的编号。
snd_ctl_dev_register()函数会在snd_card_register()中，即声卡的注册阶段被调用。注册完成后，control设备的相关信息被保存在snd_minors[]数组中，用control设备的次设备号作索引，即可在snd_minors[]数组中找出相关的信息。注册完成后的数据结构关系可以用下图进行表述：
