text: // SPDX-License-Identifier: GPL-2.0
  
  #define pr_fmt(fmt) "ACPI: " fmt
  
  #include <linux/pci.h>
  #include <linux/acpi.h>
  #include <acpi/reboot.h>
  #include <linux/delay.h>
  
  #ifdef CONFIG_PCI
  static void acpi_pci_reboot(struct acpi_generic_address *rr, u8 reset_value)
  {
  	unsigned int devfn;
  	struct pci_bus *bus0;
  
  	/* The reset register can only live on bus 0. */
  	bus0 = pci_find_bus(0, 0);
  	if (!bus0)
  		return;
  	/* Form PCI device/function pair. */
  	devfn = PCI_DEVFN((rr->address >> 32) & 0xffff,
  			  (rr->address >> 16) & 0xffff);
  	pr_debug("Resetting with ACPI PCI RESET_REG.\n");
  	/* Write the value that resets us. */
  	pci_bus_write_config_byte(bus0, devfn,
  			(rr->address & 0xffff), reset_value);
  }
  #else
  static inline void acpi_pci_reboot(struct acpi_generic_address *rr,
  				   u8 reset_value)
  {
  	pr_warn_once("PCI configuration space access is not supported\n");
  }
  #endif
  
  void acpi_reboot(void)
  {
  	struct acpi_generic_address *rr;
  	u8 reset_value;
  
  	if (acpi_disabled)
  		return;
  
  	rr = &acpi_gbl_FADT.reset_register;
  
  	/* ACPI reset register was only introduced with v2 of the FADT */
  
  	if (acpi_gbl_FADT.header.revision < 2)
  		return;
  
  	/* Is the reset register supported? The spec says we should be
  	 * checking the bit width and bit offset, but Windows ignores
  	 * these fields */
  	if (!(acpi_gbl_FADT.flags & ACPI_FADT_RESET_REGISTER))
  		return;
  
  	reset_value = acpi_gbl_FADT.reset_value;
  
  	/* The reset register can only exist in I/O, Memory or PCI config space
  	 * on a device on bus 0. */
  	switch (rr->space_id) {
  	case ACPI_ADR_SPACE_PCI_CONFIG:
  		acpi_pci_reboot(rr, reset_value);
  		break;
  
  	case ACPI_ADR_SPACE_SYSTEM_MEMORY:
  	case ACPI_ADR_SPACE_SYSTEM_IO:
  		pr_debug("ACPI MEMORY or I/O RESET_REG.\n");
  		acpi_reset();
  		break;
  	}
  
  	/*
  	 * Some platforms do not shut down immediately after writing to the
  	 * ACPI reset register, and this results in racing with the
  	 * subsequent reboot mechanism.
  	 *
  	 * The 15ms delay has been found to be long enough for the system
  	 * to reboot on the affected platforms.
  	 */
  	mdelay(15);
  }
  