mailbox: apple: Implement flush() operation

This allows clients to use the atomic-safe mailbox API style.
Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/apple-mailbox.c

@@ -17,6 +17,7 @@
  */
 
 #include <linux/apple-mailbox.h>
+#include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/gfp.h>
 #include <linux/interrupt.h>
@@ -112,6 +113,14 @@ static bool apple_mbox_hw_can_send(struct apple_mbox *apple_mbox)
 	return !(mbox_ctrl & apple_mbox->hw->control_full);
 }
 
+static bool apple_mbox_hw_send_empty(struct apple_mbox *apple_mbox)
+{
+	u32 mbox_ctrl =
+		readl_relaxed(apple_mbox->regs + apple_mbox->hw->a2i_control);
+
+	return mbox_ctrl & apple_mbox->hw->control_empty;
+}
+
 static int apple_mbox_hw_send(struct apple_mbox *apple_mbox,
 			      struct apple_mbox_msg *msg)
 {
@@ -219,6 +228,23 @@ static irqreturn_t apple_mbox_recv_irq(int irq, void *data)
 	return IRQ_HANDLED;
 }
 
+static int apple_mbox_chan_flush(struct mbox_chan *chan, unsigned long timeout)
+{
+	struct apple_mbox *apple_mbox = chan->con_priv;
+	unsigned long deadline = jiffies + msecs_to_jiffies(timeout);
+
+	while (time_before(jiffies, deadline)) {
+		if (apple_mbox_hw_send_empty(apple_mbox)) {
+			mbox_chan_txdone(&apple_mbox->chan, 0);
+			return 0;
+		}
+
+		udelay(1);
+	}
+
+	return -ETIME;
+}
+
 static int apple_mbox_chan_startup(struct mbox_chan *chan)
 {
 	struct apple_mbox *apple_mbox = chan->con_priv;
@@ -250,6 +276,7 @@ static void apple_mbox_chan_shutdown(struct mbox_chan *chan)
 
 static const struct mbox_chan_ops apple_mbox_ops = {
 	.send_data = apple_mbox_chan_send_data,
+	.flush = apple_mbox_chan_flush,
 	.startup = apple_mbox_chan_startup,
 	.shutdown = apple_mbox_chan_shutdown,
 };