mailbox: switch to hrtimer for tx_complete polling

The mailbox core uses jiffy based timer to handle polling for the
transmit completion. If the client/protocol have/support notification
of the last packet transmit completion via ACK packet, then we tick the
Tx state machine immediately in the callback. However if the client
doesn't support that mechanism we might end-up waiting for atleast a
jiffy even though the remote is ready to receive the next request.

This patch switches the timer used for that polling from jiffy-based
to hrtimer-based so that we can support polling at much higher time
resolution.

Reported-and-suggested-by: Juri Lelli <[e-mail]>
Signed-off-by: Sudeep Holla <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/mailbox.c

···
 static LIST_HEAD(mbox_cons);
 static DEFINE_MUTEX(con_mutex);
 
-static void poll_txdone(unsigned long data);
-
 static int add_to_rbuf(struct mbox_chan *chan, void *mssg)
 {
 	int idx;
···
 	spin_unlock_irqrestore(&chan->lock, flags);
 
 	if (!err && (chan->txdone_method & TXDONE_BY_POLL))
-		poll_txdone((unsigned long)chan->mbox);
+		/* kick start the timer immediately to avoid delays */
+		hrtimer_start(&chan->mbox->poll_hrt, ktime_set(0, 0),
+			      HRTIMER_MODE_REL);
 }
 
 static void tx_tick(struct mbox_chan *chan, int r)
···
 		complete(&chan->tx_complete);
 }
 
-static void poll_txdone(unsigned long data)
+static enum hrtimer_restart txdone_hrtimer(struct hrtimer *hrtimer)
 {
-	struct mbox_controller *mbox = (struct mbox_controller *)data;
+	struct mbox_controller *mbox =
+		container_of(hrtimer, struct mbox_controller, poll_hrt);
 	bool txdone, resched = false;
 	int i;
 
···
 		}
 	}
 
-	if (resched)
-		mod_timer(&mbox->poll, jiffies +
-				msecs_to_jiffies(mbox->txpoll_period));
+	if (resched) {
+		hrtimer_forward_now(hrtimer, ms_to_ktime(mbox->txpoll_period));
+		return HRTIMER_RESTART;
+	}
+	return HRTIMER_NORESTART;
 }
 
 /**
···
 		txdone = TXDONE_BY_ACK;
 
 	if (txdone == TXDONE_BY_POLL) {
-		mbox->poll.function = &poll_txdone;
-		mbox->poll.data = (unsigned long)mbox;
-		init_timer(&mbox->poll);
+		hrtimer_init(&mbox->poll_hrt, CLOCK_MONOTONIC,
+			     HRTIMER_MODE_REL);
+		mbox->poll_hrt.function = txdone_hrtimer;
 	}
 
 	for (i = 0; i < mbox->num_chans; i++) {
···
 		mbox_free_channel(&mbox->chans[i]);
 
 	if (mbox->txdone_poll)
-		del_timer_sync(&mbox->poll);
+		hrtimer_cancel(&mbox->poll_hrt);
 
 	mutex_unlock(&con_mutex);
 }

include/linux/mailbox_controller.h

···
 
 #include <linux/of.h>
 #include <linux/types.h>
-#include <linux/timer.h>
+#include <linux/hrtimer.h>
 #include <linux/device.h>
 #include <linux/completion.h>
 
···
  * @txpoll_period:	If 'txdone_poll' is in effect, the API polls for
  *			last TX's status after these many millisecs
  * @of_xlate:		Controller driver specific mapping of channel via DT
- * @poll:		API private. Used to poll for TXDONE on all channels.
+ * @poll_hrt:		API private. hrtimer used to poll for TXDONE on all
+ *			channels.
  * @node:		API private. To hook into list of controllers.
  */
 struct mbox_controller {
···
 	struct mbox_chan *(*of_xlate)(struct mbox_controller *mbox,
 				      const struct of_phandle_args *sp);
 	/* Internal to API */
-	struct timer_list poll;
+	struct hrtimer poll_hrt;
 	struct list_head node;
 };