Linux/AM5728: Questions about PRU RPMsg driver on linux

Tool/software: Linux

Hello,I tried to figure it out how A15 kicks PRU through RPMSG & mailbox,can you help me ,thanks ahead.

I read drivers/remoteproc/pru_rproc.c

static void pru_rproc_kick(struct rproc *rproc, int vq_id)
{
	struct device *dev = &rproc->dev;
	struct pru_rproc *pru = rproc->priv;
	int ret;

	dev_dbg(dev, "kicking vqid %d on PRU%d\n", vq_id, pru->id);

	if (pru->mbox) {
		/*
		 * send the index of the triggered virtqueue in the mailbox
		 * payload
		 */
		ret = mbox_send_message(pru->mbox, (void *)vq_id);
		if (ret < 0)
			dev_err(dev, "mbox_send_message failed: %d\n", ret);
	} else if (pru->irq_kick > 0) {
		ret = pruss_intc_trigger(pru->irq_kick);
		if (ret < 0)
			dev_err(dev, "pruss_intc_trigger failed: %d\n", ret);
	}
}

And I thought the driver rpmsg_pru should call "pru_rproc_kick" when it A15 send data out .

But : in drivers/rpmsg/rpmsg_pru.c("-->" means call  )

rpmsg_write() --> rmpsg_send() --> virtio_rpmsg_send --> rpmsg_send_offchannel_raw --> virtqueue_kick

I am now getting confused , question is :

1. How do the driver "rpmsg_pru.c" kicks PRU, I think the driver must access mailbox.

2.How do these drivers cooperate?  "rpmsg_pru.c",   "pru_rproc.c",   "omap-mailbox.c"