The destructor for a pcm instance is not always necessary. Since the pcm device will be released by the middle layer code automatically, you don't have to call the destructor explicitly.
The destructor would be necessary if you created special records internally and needed to release them. In such a case, set the destructor function to pcm->private_free:
Example 5.2. PCM Instance with a Destructor
static void mychip_pcm_free(struct snd_pcm *pcm)
{
struct mychip *chip = snd_pcm_chip(pcm);
/* free your own data */
kfree(chip->my_private_pcm_data);
/* do what you like else */
....
}
static int __devinit snd_mychip_new_pcm(struct mychip *chip)
{
struct snd_pcm *pcm;
....
/* allocate your own data */
chip->my_private_pcm_data = kmalloc(...);
/* set the destructor */
pcm->private_data = chip;
pcm->private_free = mychip_pcm_free;
....
}