can: c_can: c_can_poll(): only read status register after status IRQ

When the status register is read without the status IRQ pending, the
chip may not raise the interrupt line for an upcoming status interrupt
and the driver may miss a status interrupt.

It is critical that the BUSOFF status interrupt is forwarded to the
higher layers, since no more interrupts will follow without
intervention.

Thanks to Wolfgang and Joe for bringing up the first idea.
Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Joe Burmeister <joe.burmeister@devtank.co.uk>
Fixes: fa39b54c ("can: c_can: Get rid of pointless interrupts")
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/c_can/c_can.c

@@ -97,6 +97,9 @@
 #define BTR_TSEG2_SHIFT		12
 #define BTR_TSEG2_MASK		(0x7 << BTR_TSEG2_SHIFT)
 
+/* interrupt register */
+#define INT_STS_PENDING		0x8000
+
 /* brp extension register */
 #define BRP_EXT_BRPE_MASK	0x0f
 #define BRP_EXT_BRPE_SHIFT	0
@@ -1029,10 +1032,16 @@ static int c_can_poll(struct napi_struct *napi, int quota)
 	u16 curr, last = priv->last_status;
 	int work_done = 0;
 
-	priv->last_status = curr = priv->read_reg(priv, C_CAN_STS_REG);
-	/* Ack status on C_CAN. D_CAN is self clearing */
-	if (priv->type != BOSCH_D_CAN)
-		priv->write_reg(priv, C_CAN_STS_REG, LEC_UNUSED);
+	/* Only read the status register if a status interrupt was pending */
+	if (atomic_xchg(&priv->sie_pending, 0)) {
+		priv->last_status = curr = priv->read_reg(priv, C_CAN_STS_REG);
+		/* Ack status on C_CAN. D_CAN is self clearing */
+		if (priv->type != BOSCH_D_CAN)
+			priv->write_reg(priv, C_CAN_STS_REG, LEC_UNUSED);
+	} else {
+		/* no change detected ... */
+		curr = last;
+	}
 
 	/* handle state changes */
 	if ((curr & STATUS_EWARN) && (!(last & STATUS_EWARN))) {
@@ -1083,10 +1092,16 @@ static irqreturn_t c_can_isr(int irq, void *dev_id)
 {
 	struct net_device *dev = (struct net_device *)dev_id;
 	struct c_can_priv *priv = netdev_priv(dev);
+	int reg_int;
 
-	if (!priv->read_reg(priv, C_CAN_INT_REG))
+	reg_int = priv->read_reg(priv, C_CAN_INT_REG);
+	if (!reg_int)
 		return IRQ_NONE;
 
+	/* save for later use */
+	if (reg_int & INT_STS_PENDING)
+		atomic_set(&priv->sie_pending, 1);
+
 	/* disable all interrupts and schedule the NAPI */
 	c_can_irq_control(priv, false);
 	napi_schedule(&priv->napi);

drivers/net/can/c_can/c_can.h

@@ -198,6 +198,7 @@ struct c_can_priv {
 	struct net_device *dev;
 	struct device *device;
 	atomic_t tx_active;
+	atomic_t sie_pending;
 	unsigned long tx_dir;
 	int last_status;
 	u16 (*read_reg) (const struct c_can_priv *priv, enum reg index);