can: at91_can: at91_irq_err_line(): take reg_sr into account for bus off

The at91 CAN controller automatically recovers from bus-off after 128
occurrences of 11 consecutive recessive bits.

After an auto-recovered bus-off, the error counters no longer reflect
this fact. On the sam9263 the state bits in the SR register show the
current state (based on the current error counters), while on sam9x5
and newer SoCs these bits are latched.

Take any latched bus-off information from the SR register into account
when calculating the CAN new state, to start the standard CAN bus off
handling.

Link: https://lore.kernel.org/all/20231005-at91_can-rx_offload-v2-23-9987d53600e0@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/at91_can.c

@@ -437,6 +437,11 @@ static void at91_chip_start(struct net_device *dev)
 
 	priv->can.state = CAN_STATE_ERROR_ACTIVE;
 
+	/* Dummy read to clear latched line error interrupts on
+	 * sam9x5 and newer SoCs.
+	 */
+	at91_read(priv, AT91_SR);
+
 	/* Enable interrupts */
 	reg_ier = get_irq_mb_rx(priv) | AT91_IRQ_ERRP | AT91_IRQ_ERR_FRAME;
 	at91_write(priv, AT91_IER, reg_ier);
@@ -932,7 +937,7 @@ static void at91_irq_err_state(struct net_device *dev,
 	at91_write(priv, AT91_IER, reg_ier);
 }
 
-static void at91_irq_err_line(struct net_device *dev)
+static void at91_irq_err_line(struct net_device *dev, const u32 reg_sr)
 {
 	enum can_state new_state, rx_state, tx_state;
 	struct at91_priv *priv = netdev_priv(dev);
@@ -942,6 +947,23 @@ static void at91_irq_err_line(struct net_device *dev)
 
 	at91_get_berr_counter(dev, &bec);
 	can_state_get_by_berr_counter(dev, &bec, &tx_state, &rx_state);
+
+	/* The chip automatically recovers from bus-off after 128
+	 * occurrences of 11 consecutive recessive bits.
+	 *
+	 * After an auto-recovered bus-off, the error counters no
+	 * longer reflect this fact. On the sam9263 the state bits in
+	 * the SR register show the current state (based on the
+	 * current error counters), while on sam9x5 and newer SoCs
+	 * these bits are latched.
+	 *
+	 * Take any latched bus-off information from the SR register
+	 * into account when calculating the CAN new state, to start
+	 * the standard CAN bus off handling.
+	 */
+	if (reg_sr & AT91_IRQ_BOFF)
+		rx_state = CAN_STATE_BUS_OFF;
+
 	new_state = max(tx_state, rx_state);
 
 	/* state hasn't changed */
@@ -1050,7 +1072,7 @@ static irqreturn_t at91_irq(int irq, void *dev_id)
 	if (reg_sr & get_irq_mb_tx(priv))
 		at91_irq_tx(dev, reg_sr);
 
-	at91_irq_err_line(dev);
+	at91_irq_err_line(dev, reg_sr);
 
 	/* Frame Error Interrupt */
 	if (reg_sr & AT91_IRQ_ERR_FRAME)