net: enc28j60: Use threaded interrupt instead of workqueue

The Microchip ENC28J60 SPI Ethernet driver schedules a work item from
the interrupt handler because accesses to the SPI bus may sleep.

On PREEMPT_RT (which forces interrupt handling into threads) this
old-fashioned approach unnecessarily increases latency because an
interrupt results in first waking the interrupt thread, then scheduling
the work item.  So, a double indirection to handle an interrupt.

Avoid by converting the driver to modern threaded interrupt handling.
Signed-off-by: Philipp Rosenberger <p.rosenberger@kunbus.com>
Signed-off-by: Zhi Han <hanzhi09@gmail.com>
[lukas: rewrite commit message, linewrap request_threaded_irq() call]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Link: https://lore.kernel.org/r/342380d989ce26bc49f0e5d45fbb0416a5f7809f.1683606193.git.lukas@wunner.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/microchip/enc28j60.c

@@ -58,7 +58,6 @@ struct enc28j60_net {
 	struct mutex lock;
 	struct sk_buff *tx_skb;
 	struct work_struct tx_work;
-	struct work_struct irq_work;
 	struct work_struct setrx_work;
 	struct work_struct restart_work;
 	u8 bank;		/* current register bank selected */
@@ -1118,10 +1117,9 @@ static int enc28j60_rx_interrupt(struct net_device *ndev)
 	return ret;
 }
 
-static void enc28j60_irq_work_handler(struct work_struct *work)
+static irqreturn_t enc28j60_irq(int irq, void *dev_id)
 {
-	struct enc28j60_net *priv =
-		container_of(work, struct enc28j60_net, irq_work);
+	struct enc28j60_net *priv = dev_id;
 	struct net_device *ndev = priv->netdev;
 	int intflags, loop;
 
@@ -1225,6 +1223,8 @@ static void enc28j60_irq_work_handler(struct work_struct *work)
 
 	/* re-enable interrupts */
 	locked_reg_bfset(priv, EIE, EIE_INTIE);
+
+	return IRQ_HANDLED;
 }
 
 /*
@@ -1309,22 +1309,6 @@ static void enc28j60_tx_work_handler(struct work_struct *work)
 	enc28j60_hw_tx(priv);
 }
 
-static irqreturn_t enc28j60_irq(int irq, void *dev_id)
-{
-	struct enc28j60_net *priv = dev_id;
-
-	/*
-	 * Can't do anything in interrupt context because we need to
-	 * block (spi_sync() is blocking) so fire of the interrupt
-	 * handling workqueue.
-	 * Remember that we access enc28j60 registers through SPI bus
-	 * via spi_sync() call.
-	 */
-	schedule_work(&priv->irq_work);
-
-	return IRQ_HANDLED;
-}
-
 static void enc28j60_tx_timeout(struct net_device *ndev, unsigned int txqueue)
 {
 	struct enc28j60_net *priv = netdev_priv(ndev);
@@ -1559,7 +1543,6 @@ static int enc28j60_probe(struct spi_device *spi)
 	mutex_init(&priv->lock);
 	INIT_WORK(&priv->tx_work, enc28j60_tx_work_handler);
 	INIT_WORK(&priv->setrx_work, enc28j60_setrx_work_handler);
-	INIT_WORK(&priv->irq_work, enc28j60_irq_work_handler);
 	INIT_WORK(&priv->restart_work, enc28j60_restart_work_handler);
 	spi_set_drvdata(spi, priv);	/* spi to priv reference */
 	SET_NETDEV_DEV(dev, &spi->dev);
@@ -1578,7 +1561,8 @@ static int enc28j60_probe(struct spi_device *spi)
 	/* Board setup must set the relevant edge trigger type;
 	 * level triggers won't currently work.
 	 */
-	ret = request_irq(spi->irq, enc28j60_irq, 0, DRV_NAME, priv);
+	ret = request_threaded_irq(spi->irq, NULL, enc28j60_irq, IRQF_ONESHOT,
+				   DRV_NAME, priv);
 	if (ret < 0) {
 		if (netif_msg_probe(priv))
 			dev_err(&spi->dev, "request irq %d failed (ret = %d)\n",