omap2/3/4: mailbox: use dedicated work queue for handling mailbox rx interrupt

keventd_wq is a shared work-queue, and should not be used when we
need fast deterministic response.  Instead mailbox driver should
use it's own private work-queue, with it's own thread, to ensure
that handling of RX interrupts are not delayed by other drivers.

The tasklet is still used for transmission of mbox messages.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: C A Subramaniam <subramaniam.ca@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/plat-omap/mailbox.c

@@ -28,6 +28,7 @@
 
 #include <plat/mailbox.h>
 
+static struct workqueue_struct *mboxd;
 static struct omap_mbox *mboxes;
 static DEFINE_RWLOCK(mboxes_lock);
 
@@ -188,7 +189,7 @@ static void __mbox_rx_interrupt(struct omap_mbox *mbox)
 	/* no more messages in the fifo. clear IRQ source. */
 	ack_mbox_irq(mbox, IRQ_RX);
 nomem:
-	schedule_work(&mbox->rxq->work);
+	queue_work(mboxd, &mbox->rxq->work);
 }
 
 static irqreturn_t mbox_interrupt(int irq, void *p)
@@ -401,12 +402,17 @@ EXPORT_SYMBOL(omap_mbox_unregister);
 
 static int __init omap_mbox_init(void)
 {
+	mboxd = create_workqueue("mboxd");
+	if (!mboxd)
+		return -ENOMEM;
+
 	return 0;
 }
 module_init(omap_mbox_init);
 
 static void __exit omap_mbox_exit(void)
 {
+	destroy_workqueue(mboxd);
 }
 module_exit(omap_mbox_exit);