mailbox: sprd: Introduce refcnt when clients requests/free channels

Unisoc mailbox has no way to be enabled/disabled for any single channel.
They can only be set to startup or shutdown as a whole device at same time.

Add a variable to count references to avoid mailbox FIFO being reset
unexpectedly when clients are requesting or freeing channels.

Also add a lock to dismiss possible conflicts from register r/w in
different startup or shutdown threads. And fix the crash problem when early
interrupts come from channel which has not been requested by client yet.

Fixes: ca27fc26cd22 ("mailbox: sprd: Add Spreadtrum mailbox driver")
Signed-off-by: Orson Zhai <[e-mail]>
Reviewed-by: Baolin Wang <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/sprd-mailbox.c

···
 	struct clk		*clk;
 	u32			outbox_fifo_depth;
 
+	struct mutex		lock;
+	u32			refcnt;
 	struct mbox_chan	chan[SPRD_MBOX_CHAN_MAX];
 };
 
···
 		id = readl(priv->outbox_base + SPRD_MBOX_ID);
 
 		chan = &priv->chan[id];
-		mbox_chan_received_data(chan, (void *)msg);
+		if (chan->cl)
+			mbox_chan_received_data(chan, (void *)msg);
+		else
+			dev_warn_ratelimited(priv->dev,
+				    "message's been dropped at ch[%d]\n", id);
 
 		/* Trigger to update outbox FIFO pointer */
 		writel(0x1, priv->outbox_base + SPRD_MBOX_TRIGGER);
···
 	struct sprd_mbox_priv *priv = to_sprd_mbox_priv(chan->mbox);
 	u32 val;
 
-	/* Select outbox FIFO mode and reset the outbox FIFO status */
-	writel(0x0, priv->outbox_base + SPRD_MBOX_FIFO_RST);
+	mutex_lock(&priv->lock);
+	if (priv->refcnt++ == 0) {
+		/* Select outbox FIFO mode and reset the outbox FIFO status */
+		writel(0x0, priv->outbox_base + SPRD_MBOX_FIFO_RST);
 
-	/* Enable inbox FIFO overflow and delivery interrupt */
-	val = readl(priv->inbox_base + SPRD_MBOX_IRQ_MSK);
-	val &= ~(SPRD_INBOX_FIFO_OVERFLOW_IRQ | SPRD_INBOX_FIFO_DELIVER_IRQ);
-	writel(val, priv->inbox_base + SPRD_MBOX_IRQ_MSK);
+		/* Enable inbox FIFO overflow and delivery interrupt */
+		val = readl(priv->inbox_base + SPRD_MBOX_IRQ_MSK);
+		val &= ~(SPRD_INBOX_FIFO_OVERFLOW_IRQ | SPRD_INBOX_FIFO_DELIVER_IRQ);
+		writel(val, priv->inbox_base + SPRD_MBOX_IRQ_MSK);
 
-	/* Enable outbox FIFO not empty interrupt */
-	val = readl(priv->outbox_base + SPRD_MBOX_IRQ_MSK);
-	val &= ~SPRD_OUTBOX_FIFO_NOT_EMPTY_IRQ;
-	writel(val, priv->outbox_base + SPRD_MBOX_IRQ_MSK);
+		/* Enable outbox FIFO not empty interrupt */
+		val = readl(priv->outbox_base + SPRD_MBOX_IRQ_MSK);
+		val &= ~SPRD_OUTBOX_FIFO_NOT_EMPTY_IRQ;
+		writel(val, priv->outbox_base + SPRD_MBOX_IRQ_MSK);
+	}
+	mutex_unlock(&priv->lock);
 
 	return 0;
 }
···
 {
 	struct sprd_mbox_priv *priv = to_sprd_mbox_priv(chan->mbox);
 
-	/* Disable inbox & outbox interrupt */
-	writel(SPRD_INBOX_FIFO_IRQ_MASK, priv->inbox_base + SPRD_MBOX_IRQ_MSK);
-	writel(SPRD_OUTBOX_FIFO_IRQ_MASK, priv->outbox_base + SPRD_MBOX_IRQ_MSK);
+	mutex_lock(&priv->lock);
+	if (--priv->refcnt == 0) {
+		/* Disable inbox & outbox interrupt */
+		writel(SPRD_INBOX_FIFO_IRQ_MASK, priv->inbox_base + SPRD_MBOX_IRQ_MSK);
+		writel(SPRD_OUTBOX_FIFO_IRQ_MASK, priv->outbox_base + SPRD_MBOX_IRQ_MSK);
+	}
+	mutex_unlock(&priv->lock);
 }
 
 static const struct mbox_chan_ops sprd_mbox_ops = {
···
 		return -ENOMEM;
 
 	priv->dev = dev;
+	mutex_init(&priv->lock);
 
 	/*
 	 * The Spreadtrum mailbox uses an inbox to send messages to the target