can: sja1000: Prepare the use of a threaded handler

In order to support a flavor of the sja1000 which sometimes freezes, it
will be needed upon certain interrupts to perform a soft reset. The soft
reset operation takes a bit of time, so better not do it within the hard
interrupt handler but rather in a threaded handler. Let's prepare the
possibility for sja1000_err() to request "interrupting" the current flow
and request the threaded handler to be run while keeping the interrupt
line low.

There is no functional change.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/all/20230616134553.2786391-1-miquel.raynal@bootlin.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/sja1000/sja1000.c

@@ -501,7 +501,8 @@ irqreturn_t sja1000_interrupt(int irq, void *dev_id)
 	struct sja1000_priv *priv = netdev_priv(dev);
 	struct net_device_stats *stats = &dev->stats;
 	uint8_t isrc, status;
-	int n = 0;
+	irqreturn_t ret = 0;
+	int n = 0, err;
 
 	if (priv->pre_irq)
 		priv->pre_irq(priv);
@@ -546,19 +547,23 @@ irqreturn_t sja1000_interrupt(int irq, void *dev_id)
 		}
 		if (isrc & (IRQ_DOI | IRQ_EI | IRQ_BEI | IRQ_EPI | IRQ_ALI)) {
 			/* error interrupt */
-			if (sja1000_err(dev, isrc, status))
+			err = sja1000_err(dev, isrc, status);
+			if (err)
 				break;
 		}
 		n++;
 	}
 out:
+	if (!ret)
+		ret = (n) ? IRQ_HANDLED : IRQ_NONE;
+
 	if (priv->post_irq)
 		priv->post_irq(priv);
 
 	if (n >= SJA1000_MAX_IRQ)
 		netdev_dbg(dev, "%d messages handled in ISR", n);
 
-	return (n) ? IRQ_HANDLED : IRQ_NONE;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(sja1000_interrupt);