linux-kernel - [PATCH 7/7] serial: kgdb_nmi: Replace hrtimer with irq

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Date:   Mon, 22 Jun 2020 19:56:24 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, daniel.thompson@...aro.org,
        jason.wessel@...driver.com, dianders@...omium.org, jslaby@...e.com,
        linux@...linux.org.uk, linux-kernel@...r.kernel.org,
        Sumit Garg <sumit.garg@...aro.org>
Subject: [PATCH 7/7] serial: kgdb_nmi: Replace hrtimer with irq_work ping

From: Daniel Thompson <daniel.thompson@...aro.org>

Currently kgdb_nmi relies on a 100Hz timer to "defer" work from an NMI
handler. This is essentially doing the job that irq_work is designed to
do but in an odd and inefficient manner. Remove the timer code and
replace it with more appropriate irq_work related APIs.

Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
---
 drivers/tty/serial/kgdb_nmi.c | 35 ++++++++++-------------------------
 1 file changed, 10 insertions(+), 25 deletions(-)

diff --git a/drivers/tty/serial/kgdb_nmi.c b/drivers/tty/serial/kgdb_nmi.c
index 2580f39..ad84eef 100644
--- a/drivers/tty/serial/kgdb_nmi.c
+++ b/drivers/tty/serial/kgdb_nmi.c
@@ -21,7 +21,7 @@
 #include <linux/tty_flip.h>
 #include <linux/serial_core.h>
 #include <linux/interrupt.h>
-#include <linux/hrtimer.h>
+#include <linux/irq_work.h>
 #include <linux/tick.h>
 #include <linux/kfifo.h>
 #include <linux/kgdb.h>
@@ -127,7 +127,7 @@ static struct console kgdb_nmi_console = {
 
 struct kgdb_nmi_tty_priv {
 	struct tty_port port;
-	struct timer_list timer;
+	struct irq_work work;
 	STRUCT_KFIFO(char, KGDB_NMI_FIFO_SIZE) fifo;
 };
 
@@ -141,13 +141,12 @@ static void kgdb_tty_recv(int ch)
 	if (!kgdb_nmi_port || ch < 0)
 		return;
 	/*
-	 * Can't use port->tty->driver_data as tty might be not there. Timer
-	 * will check for tty and will get the ref, but here we don't have to
-	 * do that, and actually, we can't: we're in NMI context, no locks are
-	 * possible.
+	 * Can't use port->tty->driver_data as tty might be not there. Also, we
+	 * can't even do that as we're in NMI context, no locks are possible.
 	 */
 	priv = container_of(kgdb_nmi_port, struct kgdb_nmi_tty_priv, port);
 	kfifo_in(&priv->fifo, &c, 1);
+	irq_work_queue(&priv->work);
 }
 
 static int kgdb_nmi_poll_one_knock(void)
@@ -222,18 +221,12 @@ bool kgdb_nmi_poll_knock(void)
 	return true;
 }
 
-/*
- * The tasklet is cheap, it does not cause wakeups when reschedules itself,
- * instead it waits for the next tick.
- */
-static void kgdb_nmi_tty_receiver(struct timer_list *t)
+static void kgdb_nmi_tty_receiver(struct irq_work *work)
 {
-	struct kgdb_nmi_tty_priv *priv = from_timer(priv, t, timer);
+	struct kgdb_nmi_tty_priv *priv =
+	    container_of(work, struct kgdb_nmi_tty_priv, work);
 	char ch;
 
-	priv->timer.expires = jiffies + (HZ/100);
-	add_timer(&priv->timer);
-
 	if (likely(!atomic_read(&kgdb_nmi_num_readers) ||
 		   !kfifo_len(&priv->fifo)))
 		return;
@@ -245,22 +238,13 @@ static void kgdb_nmi_tty_receiver(struct timer_list *t)
 
 static int kgdb_nmi_tty_activate(struct tty_port *port, struct tty_struct *tty)
 {
-	struct kgdb_nmi_tty_priv *priv =
-	    container_of(port, struct kgdb_nmi_tty_priv, port);
-
 	kgdb_nmi_port = port;
-	priv->timer.expires = jiffies + (HZ/100);
-	add_timer(&priv->timer);
 
 	return 0;
 }
 
 static void kgdb_nmi_tty_shutdown(struct tty_port *port)
 {
-	struct kgdb_nmi_tty_priv *priv =
-	    container_of(port, struct kgdb_nmi_tty_priv, port);
-
-	del_timer(&priv->timer);
 	kgdb_nmi_port = NULL;
 }
 
@@ -279,7 +263,8 @@ static int kgdb_nmi_tty_install(struct tty_driver *drv, struct tty_struct *tty)
 		return -ENOMEM;
 
 	INIT_KFIFO(priv->fifo);
-	timer_setup(&priv->timer, kgdb_nmi_tty_receiver, 0);
+	init_irq_work(&priv->work, kgdb_nmi_tty_receiver);
+	atomic_set(&priv->work.flags, IRQ_WORK_LAZY);
 	tty_port_init(&priv->port);
 	priv->port.ops = &kgdb_nmi_tty_port_ops;
 	tty->driver_data = priv;
-- 
2.7.4