[C++知识库]control设备的注册流程

我们知道，control设备的创建一般是在ASoC-codec层完成，然后是在注册声卡的时候注册。我们先来找到在声卡注册时执行的函数 snd_soc_add_card_controls() 

int snd_soc_add_card_controls(struct snd_soc_card *soc_card,
        const struct snd_kcontrol_new *controls, int num_controls)
{
        struct snd_card *card = soc_card->snd_card;

        return snd_soc_add_controls(card, soc_card->dev, controls, num_controls,
                        NULL, soc_card);
}

可以看到这个函数主要是执行了snd_soc_add_controls()这个函数

static int snd_soc_add_controls(struct snd_card *card, struct device *dev,
        const struct snd_kcontrol_new *controls, int num_controls,
        const char *prefix, void *data)
{
        int err, i;

        for (i = 0; i < num_controls; i++) {
                const struct snd_kcontrol_new *control = &controls[i];

                err = snd_ctl_add(card, snd_soc_cnew(control, data,
                                                     control->name, prefix));
                if (err < 0) {
                        dev_err(dev, "ASoC: Failed to add %s: %d\n",
                                control->name, err);
                        return err;
                }
        }

        return 0;
}

这里是利用num_controls来进行循环次数判定，循环内容主要是执行了snd_ctl_add()，再经过层层调用，最后来到

/* 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;
        unsigned int count;
        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;

        list_add_tail(&kcontrol->list, &card->controls);
        card->controls_count += kcontrol->count;
        kcontrol->id.numid = card->last_numid + 1;
        card->last_numid += kcontrol->count;

        id = kcontrol->id;
        count = kcontrol->count;
        for (idx = 0; idx < count; idx++, id.index++, id.numid++)
                snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_ADD, &id);

        return 0;
}

1. 找到control，然后判断是代替还是新增，代替则删除旧的control

2. 将control添加到card的链表，并对card中与control相关的参数进行更新

3. 设置读取节点时的相关配置