drivers/pci/setup-irq.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Support routines for initializing a PCI subsystem
 *
 * Extruded from code written by
 *      Dave Rusling (david.rusling@reo.mts.dec.com)
 *      David Mosberger (davidm@cs.arizona.edu)
 *	David Miller (davem@redhat.com)
 */

#include <linux/kernel.h>
#include <linux/pci.h>
#include <linux/errno.h>
#include <linux/ioport.h>
#include <linux/cache.h>
#include "pci.h"

void pci_assign_irq(struct pci_dev *dev)
{
	u8 pin;
	u8 slot = -1;
	int irq = 0;
	struct pci_host_bridge *hbrg = pci_find_host_bridge(dev->bus);

	if (!(hbrg->map_irq)) {
		pci_dbg(dev, "runtime IRQ mapping not provided by arch\n");
		return;
	}

	/*
	 * If this device is not on the primary bus, we need to figure out
	 * which interrupt pin it will come in on. We know which slot it
	 * will come in on because that slot is where the bridge is. Each
	 * time the interrupt line passes through a PCI-PCI bridge we must
	 * apply the swizzle function.
	 */
	pci_read_config_byte(dev, PCI_INTERRUPT_PIN, &pin);
	/* Cope with illegal. */
	if (pin > 4)
		pin = 1;

	if (pin) {
		/* Follow the chain of bridges, swizzling as we go. */
		if (hbrg->swizzle_irq)
			slot = (*(hbrg->swizzle_irq))(dev, &pin);

		/*
		 * If a swizzling function is not used, map_irq() must
		 * ignore slot.
		 */
		irq = (*(hbrg->map_irq))(dev, slot, pin);
		if (irq == -1)
			irq = 0;
	}
	dev->irq = irq;

	pci_dbg(dev, "assign IRQ: got %d\n", dev->irq);

	/*
	 * Always tell the device, so the driver knows what is the real IRQ
	 * to use; the device does not use it.
	 */
	pci_write_config_byte(dev, PCI_INTERRUPT_LINE, irq);
}