[alsa-devel] Why is substream->pcm->dev == NULL?

In my _pointer callback function, I have this:

printk(KERN_INFO "%s:%u substream->pcm->dev=%p, substream->pcm->card->dev=%p\n", __func__, __LINE__, substream->pcm->dev, substream->pcm->card->dev);

During playback, it displays this:

fsl_dma_pointer:698 substream->pcm->dev=00000000, substream->pcm->card->dev=dfa50a08

Why is substream->pcm->dev == NULL? What's the point of having this field if you're not going to use it?