danos/system/kernel/acpi.zig

//! ACPI discovery backend.
//!
//! Walks the ACPI tables the firmware left in memory (starting from the RSDP the
//! bootloader handed us) and translates the static tables into the generic
//! `device` model, so the kernel enumerates hardware without knowing ACPI is the
//! source. This is deliberately the *static-table* path, and **only** that: MADT
//! (CPUs / interrupt controllers), MCFG (PCIe ECAM -> PCI enumeration), HPET
//! (timer), and FADT (power register map). The DSDT/SSDT bytecode is *not*
//! interpreted here — the kernel collects the blobs and publishes them on the
//! acpi-tables node for the ring-3 acpi service to parse (device enumeration and
//! soft-off). Keeping the ~0.5 MB AML interpretation out of kernel init keeps it
//! off the single-core critical path (nothing else runs alongside it there).
//!
//! ACPI tables live in `.acpi_tables` / `.acpi_nvs` memory, which the kernel
//! identity-maps, so table addresses are dereferenced directly. PCIe ECAM is MMIO
//! and is *not* mapped up front, so configuration-space pages are mapped on demand via
//! the `Hal.mapMmio` callback the caller supplies (the architecture VMM's map primitive).

const std = @import("std");
const boot_handoff = @import("boot-handoff");
const abi = @import("abi");
const parameters = @import("parameters");
const device_model = @import("device-model.zig");
const DeviceTree = device_model.DeviceTree;
const Hal = device_model.Hal;

/// A hardware register located either in MMIO or I/O-port space, as ACPI's
/// Generic Address Structure describes. `address == 0` means "not present".
pub const RegisterAccess = struct {
    /// true = system memory (MMIO), false = system I/O port space.
    mmio: bool = false,
    address: u64 = 0,
    /// Access width in bytes.
    width: u8 = 0,

    pub fn present(self: RegisterAccess) bool {
        return self.address != 0;
    }
};

/// The power register map, extracted from the FADT during discovery. Populated by
/// `discover`, read by `power` (kernel reboot). The **sleep-state (`_Sx`) values
/// live in AML**, which the kernel no longer parses — soft-off (S5) is owned by the
/// ring-3 acpi service (it re-parses the blobs on the published acpi-tables node and
/// writes the PM1 control register itself). So this holds only the FADT scalars.
pub const PowerInformation = struct {
    /// The System Control Interrupt's GSI (FADT SCI_INT) — the line ACPI events
    /// (power button, GPEs) arrive on. Published to the acpi service for M21.
    sci_interrupt: u16 = 0,
    /// The SMM command port and the value that switches the platform into ACPI mode.
    smi_cmd: u16 = 0,
    acpi_enable: u8 = 0,
    acpi_disable: u8 = 0,
    /// PM1 control registers — writing SLP_TYP|SLP_EN here enters a sleep state.
    pm1a_cnt: RegisterAccess = .{},
    pm1b_cnt: RegisterAccess = .{},
    /// The FADT reset register and the value to write to it.
    reset: RegisterAccess = .{},
    reset_value: u8 = 0,
    reset_supported: bool = false,
};

/// Filled in by `discover`; `reboot` (below) reads it to restart the machine.
pub var power_information: PowerInformation = .{};

/// A legacy ISA IRQ remapped to a different global system interrupt (GSI), from a
/// MADT Interrupt Source Override. `flags` are the MPS INTI polarity/trigger bits.
pub const IsoEntry = struct {
    source: u8,
    gsi: u32,
    flags: u16,
};

/// Firmware facts the architecture layer needs to avoid legacy assumptions (so danos boots
/// on legacy-free UEFI Class 3 machines). MMIO device *addresses* (HPET, IOAPIC)
/// come from the device tree instead; this holds the scalar facts that have no
/// natural device node.
pub const PlatformInformation = struct {
    /// Whether the legacy 8259 PIC is present (MADT flags bit 0, PCAT_COMPAT). When
    /// false, the PIC must not be programmed (it may not exist).
    pic_present: bool = false,
    /// Local APIC MMIO base (MADT, honouring a type-5 address override).
    lapic_base: u64 = 0xFEE00000,
    /// The ACPI power-management timer — a fixed 3.579545 MHz counter usable as a
    /// calibration reference when no HPET is present.
    pm_timer: RegisterAccess = .{},
    /// true = 32-bit PM timer counter, false = 24-bit (FADT flag TMR_VALUE_EXT).
    pm_timer_32bit: bool = false,
    /// The console UART the firmware points at (SPCR), if any — MMIO or I/O port.
    spcr_uart: ?RegisterAccess = null,
    /// SPCR interface type (0/1 = 16550/16450, …).
    spcr_kind: u8 = 0,
    /// ISA-IRQ-to-GSI remappings from the MADT (for future IOAPIC routing).
    overrides: [16]IsoEntry = undefined,
    override_count: usize = 0,
    /// Whether an IOMMU (VT-d DMA-remapping unit) was found in the ACPI DMAR table.
    /// When false, `device_claim` on a DMA-capable device is equivalent to granting
    /// ring 0 — a device can DMA to any physical address (docs/driver-model.md M16),
    /// and the kernel says so at every boot (the fail-open platform log line). When
    /// true, the IOMMU core builds per-device translation domains from this record.
    iommu_present: bool = false,
    /// true when the present unit is AMD-Vi (from IVRS) rather than Intel VT-d (DMAR).
    /// The two are mutually exclusive on real hardware; the IOMMU core picks the backend.
    iommu_is_amd: bool = false,
    /// MMIO base of the selected DMA-remapping hardware unit — the VT-d DRHD with
    /// INCLUDE_PCI_ALL (the catch-all unit; falls back to the first), or the AMD-Vi
    /// IOMMU's control-register base from the first IVHD.
    iommu_base: u64 = 0,
    /// Whether the selected unit carries INCLUDE_PCI_ALL. False means every unit is
    /// device-scoped (unusual) — the core still enables on the selected unit but
    /// devices outside its scope remain untranslated.
    iommu_include_all: bool = false,
    /// DRHD units in the DMAR beyond the selected one. Devices scoped to those units
    /// (typically the integrated GPU) are NOT translated by v1 — the boot log warns.
    iommu_extra_units: u8 = 0,
    /// Reserved-memory regions (DMAR RMRRs): firmware-owned buffers a named device
    /// keeps DMAing into across the OS handoff (classically the xHC keyboard-emulation
    /// buffer). These must be identity-mapped in the device's domain BEFORE translation
    /// enables, or platform firmware breaks. Only single-path endpoint scopes are
    /// recorded; anything fancier is skipped with a loud log at parse time.
    rmrr: [maximum_rmrr]RmrrRegion = undefined,
    rmrr_count: usize = 0,
    /// RMRR device scopes the parser could not record (multi-hop paths, sub-hierarchy
    /// types, or table overflow). Non-zero means a device keeps an unmapped firmware
    /// buffer — the kernel boot log warns loudly (the platform module itself is
    /// log-free by design; it records, the kernel reports).
    rmrr_skipped: u8 = 0,
};

pub const maximum_rmrr = 8;

/// One recorded RMRR: the device (requester id) and the inclusive physical range it
/// must always be allowed to reach.
pub const RmrrRegion = struct {
    bdf: u16,
    base: u64,
    limit: u64,
};

/// Filled in by `discover`; the architecture layer reads it during bring-up.
pub var platform_information: PlatformInformation = .{};

/// One usable logical processor, from a MADT type-0 (Local APIC) record. The
/// `apic_id` is the Local APIC ID that SMP bring-up targets to wake this core
/// (INIT–SIPI–SIPI); `processor_id` is the ACPI namespace handle. Only processors
/// the firmware marks *enabled* are recorded — a disabled one can't be started.
pub const Cpu = struct {
    processor_id: u8,
    apic_id: u8,
    /// MADT flags bit 1: usable but firmware-started offline (hot-plug / deferred
    /// bring-up), as opposed to already available. Informational for now.
    online_capable: bool,
};

/// The set of usable logical processors the MADT listed — the hardware's degree of
/// parallelism. Includes the bootstrap processor danos already runs on; the rest
/// are the application processors SMP bring-up would start (see docs/smp.md).
pub const CpuInformation = struct {
    /// A static pool sized well above any danos target (a desktop, two 4-core Pis).
    /// If the MADT ever lists more, the surplus is dropped and counted in `dropped`
    /// so the truncation is never silent.
    cpus: [maximum_cpus]Cpu = undefined,
    count: usize = 0,
    dropped: usize = 0,
};

const maximum_cpus = parameters.maximum_cpus;

/// Filled in by `discover` (from the MADT); SMP bring-up reads it to wake the APs.
pub var cpu_information: CpuInformation = .{};

/// Physical address of the DSDT the FADT points at, or 0.
pub var dsdt_physical: u64 = 0;

/// The FADT itself (physical + length), published on the acpi-tables node so
/// the ring-3 acpi service can read the PM1 event and GPE blocks it needs for
/// the event side (docs/acpi.md — ACPI events). Distinguished from the AML
/// blob resources by its intact "FACP" header — the blobs are header-stripped.
var fadt_physical: u64 = 0;
var fadt_length: u64 = 0;

// AML blocks (DSDT + any SSDTs) collected during the table walk, as physical
// address + length of each table's post-header bytecode. The kernel does not
// interpret them — it publishes them on the acpi-tables node for the ring-3 acpi
// service to parse (device enumeration + soft-off). See publishAcpiTablesNode.
var aml_block_physical: [32]u64 = undefined;
var aml_block_len: [32]usize = undefined;
var aml_block_count: usize = 0;

fn addAmlBlock(sdt_physical: u64) void {
    if (aml_block_count >= aml_block_physical.len or sdt_physical == 0) return;
    const h: *const SystemDescriptorTableHeader = @ptrFromInt(boot_handoff.physicalToVirtual(sdt_physical));
    if (h.length <= @sizeOf(SystemDescriptorTableHeader)) return;
    aml_block_physical[aml_block_count] = sdt_physical + @sizeOf(SystemDescriptorTableHeader);
    aml_block_len[aml_block_count] = h.length - @sizeOf(SystemDescriptorTableHeader);
    aml_block_count += 1;
}

/// RSDP structure for revision 0 (version 1.0)
const RootSystemDescriptionPointer = extern struct {
    /// An 8 byte magic number used for locating the RSDP, containing RSD PTR.
    signature: [8]u8,
    /// A byte used to verify the first 20 bytes of the RSDP
    checksum: u8,
    /// An OEM-supplied string that identified the OEM.
    oem_id: [6]u8,
    /// The RSDP revision, used for determining which fields are available.
    revision: u8,
    /// A 32-bit physical address pointing to the RSDT.
    root_system_description_table_address: u32 align(1),
};

/// XSDP structure for revision 2 (version 2.0+)
const ExtendedSystemDescriptorPointer = extern struct {
    /// An 8 byte magic number used for locating the RSDP, containing RSD PTR.
    signature: [8]u8,
    /// A byte used to verify the first 20 bytes of the RSDP
    checksum: u8,
    /// An OEM-supplied string that identified the OEM.
    oem_id: [6]u8,
    /// The RSDP revision, used for determining which fields are available.
    revision: u8,
    /// deprecated since version 2.0. A 32-bit physical address pointing to the RSDT.
    root_system_description_table_address: u32 align(1),
    /// The size of the RSDP.
    length: u32 align(1),
    /// A 64-bit physical address pointing to the XSDT. If the revision is at least 2, the XSDT
    /// should be used regardless of architecture, as the RSDT was deprecated.
    extended_system_descriptor_table_address: u64 align(1),
    /// A checksum used for the entire table.
    extended_checksum: u8,
    reserved: [3]u8,
};

/// Multiple APIC Description Table (MADT)
const APIC: [4]u8 = "APIC".*;
/// Boot Error Record Table (BERT)
const BERT: [4]u8 = "BERT".*;
/// Corrected Platform Error Polling Table (CPEP)
const CPEP: [4]u8 = "CPEP".*;
/// Differentiated System Description Table (DSDT)
const DSDT: [4]u8 = "DSDT".*;
/// Embedded Controller Boot Resources Table (ECDT)
const ECDT: [4]u8 = "ECDT".*;
/// Error Injection Table (EINJ)
const EINJ: [4]u8 = "EINJ".*;
/// Error Record Serialization Table (ERST)
const ERST: [4]u8 = "ERST".*;
/// Fixed ACPI Description Table (FADT)
const FACP: [4]u8 = "FACP".*;
/// Firmware ACPI Control Structure (FACS)
const FACS: [4]u8 = "FACS".*;
/// Hardware Error Source Table (HEST)
const HEST: [4]u8 = "HEST".*;
/// High Precision Event Timer table (HPET)
const HPET: [4]u8 = "HPET".*;
/// PCI Express memory-mapped configuration space table (MCFG)
const MCFG: [4]u8 = "MCFG".*;
/// Maximum System Characteristics Table (MSCT)
const MSCT: [4]u8 = "MSCT".*;
/// Memory Power State Table (MPST)
const MPST: [4]u8 = "MPST".*;
//  Platform Memory Topology Table (PMTT)
const PMTT: [4]u8 = "PMTT".*;
/// Persistent System Description Table (PSDT)
const PSDT: [4]u8 = "PSDT".*;
/// ACPI RAS Feature Table (RASF)
const RASF: [4]u8 = "RASF".*;
/// Root System Description Table
const RSDT: [4]u8 = "RSDT".*;
/// Smart Battery Specification Table (SBST)
const SBST: [4]u8 = "SBST".*;
/// System Locality System Information Table (SLIT)
const SLIT: [4]u8 = "SLIT".*;
/// System Resource Affinity Table (SRAT)
const SRAT: [4]u8 = "SRAT".*;
/// Secondary System Description Table (SSDT)
const DMAR: [4]u8 = "DMAR".*;
/// I/O Virtualization Reporting Structure (IVRS) — the AMD-Vi analogue of DMAR.
const IVRS: [4]u8 = "IVRS".*;
const SSDT: [4]u8 = "SSDT".*;
/// Serial Port Console Redirection table (SPCR) — the firmware's console UART.
const SPCR: [4]u8 = "SPCR".*;
/// Extended System Description Table (XSDT; 64-bit version of the RSDT)
const XSDT: [4]u8 = "XSDT".*;

/// The header every system descriptor table (RSDT/XSDT and each SDT) begins with.
const SystemDescriptorTableHeader = extern struct {
    /// A 4 byte signature used for identification (e.g. "RSDT", "APIC").
    signature: [4]u8,
    /// The length of the entire table, including the header.
    length: u32 align(1),
    /// The revision of the ACPI spec this table conforms to.
    revision: u8,
    /// An 8-bit checksum field for the whole table, inclusive of the header.
    checksum: u8,
    /// An OEM-supplied string that identified the OEM.
    oem_id: [6]u8,
    oem_table_id: [8]u8,
    oem_revision: u32 align(1),
    creator_id: u32 align(1),
    creator_revision: u32 align(1),
};

// --- MADT: Multiple APIC Description Table (signature "APIC") ---------------

const Madt = extern struct {
    header: SystemDescriptorTableHeader,
    local_apic_address: u32 align(1),
    flags: u32 align(1),
    // Followed by a variable-length run of interrupt-controller records, each a
    // MadtRecordHeader plus a type-specific body.
};

const MadtRecordHeader = extern struct {
    type: u8,
    length: u8,
};

/// MADT record type 0: a processor's Local APIC.
const MadtLocalApic = extern struct {
    record: MadtRecordHeader,
    processor_id: u8,
    apic_id: u8,
    /// bit 0 = enabled, bit 1 = online-capable.
    flags: u32 align(1),
};

/// MADT record type 1: an I/O APIC.
const MadtIoApic = extern struct {
    record: MadtRecordHeader,
    io_apic_id: u8,
    reserved: u8,
    address: u32 align(1),
    /// First global system interrupt this I/O APIC handles.
    gsi_base: u32 align(1),
};

/// MADT record type 2: an Interrupt Source Override (ISA IRQ -> GSI remap).
const MadtIso = extern struct {
    record: MadtRecordHeader,
    bus: u8,
    source: u8,
    gsi: u32 align(1),
    flags: u16 align(1),
};

/// MADT record type 5: Local APIC Address Override (64-bit MMIO base).
const MadtLapicOverride = extern struct {
    record: MadtRecordHeader,
    reserved: u16 align(1),
    address: u64 align(1),
};

// --- MCFG: PCIe ECAM configuration space (signature "MCFG") -----------------

const Mcfg = extern struct {
    header: SystemDescriptorTableHeader,
    reserved: u64 align(1),
    // Followed by one or more McfgAllocation entries.
};

const McfgAllocation = extern struct {
    /// Physical base of this segment group's ECAM window.
    base_address: u64 align(1),
    segment_group: u16 align(1),
    start_bus: u8,
    end_bus: u8,
    reserved: u32 align(1),
};

// --- HPET (signature "HPET") ------------------------------------------------

const Hpet = extern struct {
    header: SystemDescriptorTableHeader,
    hardware_rev_id: u8,
    flags: u8,
    pci_vendor_id: u16 align(1),
    // Generic Address Structure describing the register block.
    address_space_id: u8,
    register_bit_width: u8,
    register_bit_offset: u8,
    gas_reserved: u8,
    address: u64 align(1),
    hpet_number: u8,
    minimum_tick: u16 align(1),
    page_protection: u8,
};

// --- Entry point ------------------------------------------------------------

/// Discover hardware from the ACPI tables rooted at `rsdp_physical` and populate
/// `device_tree`. `hal` provides MMIO mapping (for PCIe ECAM) and port I/O. Also parses the
/// FADT into `power_information`, and publishes the AML blobs for the ring-3 acpi service.
pub fn discover(rsdp_physical: u64, memory_regions: []const boot_handoff.MemoryRegion, device_tree: *DeviceTree, hal: Hal) !void {
    if (rsdp_physical == 0) return error.NoRsdp;
    boot_memory_regions = memory_regions;

    // Start clean so a re-run doesn't accumulate stale state.
    power_information = .{};
    fadt_physical = 0;
    fadt_length = 0;
    platform_information = .{};
    dsdt_physical = 0;
    aml_block_count = 0;

    const rsdp: *const RootSystemDescriptionPointer = @ptrFromInt(boot_handoff.physicalToVirtual(rsdp_physical));
    if (!std.mem.eql(u8, &rsdp.signature, "RSD PTR ")) return error.BadRsdpSignature;
    // Revision 0 checksums only the first 20 bytes (the v1.0 RSDP).
    if (!checksumOk(@ptrFromInt(boot_handoff.physicalToVirtual(rsdp_physical)), 20)) return error.BadRsdpChecksum;

    if (rsdp.revision >= 2) {
        const xsdp: *const ExtendedSystemDescriptorPointer = @ptrFromInt(boot_handoff.physicalToVirtual(rsdp_physical));
        if (!checksumOk(@ptrFromInt(boot_handoff.physicalToVirtual(rsdp_physical)), xsdp.length)) return error.BadXsdpChecksum;
        try walkRoot(u64, xsdp.extended_system_descriptor_table_address, device_tree, hal);
    } else {
        try walkRoot(u32, rsdp.root_system_description_table_address, device_tree, hal);
    }

    // The kernel does **not** interpret the DSDT/SSDTs. Static-table discovery
    // above (MADT/HPET/FADT/MCFG) is all the kernel needs — CPUs, timers, PCIe,
    // and the power register map. The AML bytecode (device enumeration and the
    // sleep-state `_Sx` values for soft-off) is entirely the ring-3 acpi service's
    // job: it claims the acpi-tables node published below, parses the same blobs,
    // and both registers the `_HID` devices and owns S5. Not parsing ~0.5 MB of
    // AML in the kernel keeps boot latency off the critical, single-core path.

    // Publish the acpi-tables node (docs/discovery.md): the AML blobs as
    // memory resources for the acpi service to map and parse in ring 3, a broad
    // io_port grant for the OperationRegion access its interpreter needs, and
    // the SCI for the events track (M21). Exactly one node, one trusted
    // claimant — the sole path by which AML (devices + soft-off) reaches ring 3,
    // now that the kernel keeps only the *static* tables for itself.
    publishAcpiTablesNode(device_tree) catch {};
}

/// Build the acpi-tables node (see the call site in discover). Best-effort: a
/// failure here leaves the kernel-seeded tree working, only the ring-3 service
/// finds nothing to claim.
fn publishAcpiTablesNode(device_tree: *DeviceTree) !void {
    const node = try device_tree.addChild(device_tree.root, .acpi_tables, "acpi-tables");
    // One memory resource per AML block — page-aligned base down, length padded
    // up to cover the bytecode, so mmio_map hands the service a pointer into it.
    var i: usize = 0;
    while (i < aml_block_count and i < device_model.maximum_resources - 2) : (i += 1) {
        // mmio_map preserves the sub-page offset, so the service maps this and
        // gets a pointer straight to the bytecode.
        _ = node.addResource(.memory, aml_block_physical[i], aml_block_len[i]);
    }
    // The broad I/O grant: OperationRegions name whatever ports the firmware
    // chose (EC, PM1, GPE, SMBus); which ports cannot be known before the AML
    // that names them is parsed, so the grant is the whole space — the honest
    // trust boundary of docs/discovery.md (the acpi service's one trusted node).
    _ = node.addResource(.io_port, 0, 1 << 16);
    // A broad interrupt window: ACPI _CRS names legacy ISA IRQs (the PS/2 lines
    // 1 and 12, the RTC, …), and the service registers those devices under this
    // node, so it must own a superset. The range [0, 256) covers every GSI; the
    // SCI (recorded first, len 1) stays distinct so M21 can pick it out.
    if (power_information.sci_interrupt != 0) _ = node.addResource(.irq, power_information.sci_interrupt, 1);
    _ = node.addResource(.irq, 0, 256);
    // The FADT rides along (M21): the service reads the PM1 event / GPE blocks
    // from its own copy, telling it apart from the AML blobs by signature.
    if (fadt_physical != 0) _ = node.addResource(.memory, fadt_physical, fadt_length);
}

/// Walk the RSDT (Entry = u32) or XSDT (Entry = u64): validate it, then dispatch
/// each SDT it points at. A bad individual table is skipped, not fatal.
fn walkRoot(comptime Entry: type, root_physical: u64, device_tree: *DeviceTree, hal: Hal) !void {
    const header: *const SystemDescriptorTableHeader = @ptrFromInt(boot_handoff.physicalToVirtual(root_physical));
    if (!checksumOk(@ptrFromInt(boot_handoff.physicalToVirtual(root_physical)), header.length)) return error.BadRootChecksum;

    const count = (header.length - @sizeOf(SystemDescriptorTableHeader)) / @sizeOf(Entry);
    const base: [*]const u8 = @ptrFromInt(boot_handoff.physicalToVirtual(root_physical));
    const entries: [*]align(1) const Entry = @ptrCast(base + @sizeOf(SystemDescriptorTableHeader));

    for (entries[0..count]) |ent| {
        const sdt_physical: u64 = ent; // u32 entries widen; u64 pass through
        handleTable(device_tree, hal, sdt_physical) catch continue;
    }
}

/// Dispatch a single SDT on its signature.
fn handleTable(device_tree: *DeviceTree, hal: Hal, sdt_physical: u64) !void {
    const header: *const SystemDescriptorTableHeader = @ptrFromInt(boot_handoff.physicalToVirtual(sdt_physical));
    const sig = header.signature;
    if (std.mem.eql(u8, &sig, &APIC)) {
        try parseMadt(device_tree, header);
    } else if (std.mem.eql(u8, &sig, &MCFG)) {
        try parseMcfg(device_tree, header);
    } else if (std.mem.eql(u8, &sig, &HPET)) {
        try parseHpet(device_tree, hal, header);
    } else if (std.mem.eql(u8, &sig, &FACP)) {
        fadt_physical = sdt_physical;
        fadt_length = header.length;
        parseFadt(header);
    } else if (std.mem.eql(u8, &sig, &SPCR)) {
        parseSpcr(header);
    } else if (std.mem.eql(u8, &sig, &DMAR)) {
        parseDmar(header);
    } else if (std.mem.eql(u8, &sig, &IVRS)) {
        parseIvrs(header);
    } else if (std.mem.eql(u8, &sig, &SSDT)) {
        // Secondary namespace bytecode — collect it to publish for the ring-3 parse.
        addAmlBlock(sdt_physical);
    }
    // Any other signature is recognised but left opaque for now.
}

/// MADT -> one processor node per Local APIC, one interrupt_controller per I/O APIC.
fn parseMadt(device_tree: *DeviceTree, header: *const SystemDescriptorTableHeader) !void {
    const madt: *const Madt = @ptrCast(header);
    const total: usize = header.length;
    const base: [*]const u8 = @ptrCast(header);
    var ioapic_index: usize = 0;

    // MADT header: local APIC base + flags (bit 0 = 8259 PIC present).
    platform_information.lapic_base = madt.local_apic_address;
    platform_information.pic_present = madt.flags & 1 != 0;

    var off: usize = @sizeOf(Madt);
    while (off + @sizeOf(MadtRecordHeader) <= total) {
        const rec: *const MadtRecordHeader = @ptrCast(base + off);
        if (rec.length < @sizeOf(MadtRecordHeader)) break; // malformed; avoid a spin
        switch (rec.type) {
            0 => {
                const la: *const MadtLocalApic = @ptrCast(base + off);
                // bit 0 = enabled: skip processors the firmware marks unusable.
                if (la.flags & 1 != 0) {
                    var nb: [24]u8 = undefined;
                    const nm = std.fmt.bufPrint(&nb, "cpu{d}", .{la.processor_id}) catch "cpu";
                    _ = try device_tree.addChild(device_tree.root, .processor, nm);
                    // Also record it as a schedulable core (with the APIC ID an AP
                    // wake needs, which the device node name doesn't preserve).
                    if (cpu_information.count < cpu_information.cpus.len) {
                        cpu_information.cpus[cpu_information.count] = .{
                            .processor_id = la.processor_id,
                            .apic_id = la.apic_id,
                            .online_capable = la.flags & 2 != 0,
                        };
                        cpu_information.count += 1;
                    } else {
                        cpu_information.dropped += 1;
                    }
                }
            },
            1 => {
                const io: *const MadtIoApic = @ptrCast(base + off);
                var nb: [24]u8 = undefined;
                const nm = std.fmt.bufPrint(&nb, "ioapic{d}", .{ioapic_index}) catch "ioapic";
                ioapic_index += 1;
                const d = try device_tree.addChild(device_tree.root, .interrupt_controller, nm);
                _ = d.addResource(.memory, io.address, 0x20);
                // The GSI range this I/O APIC handles, starting at gsi_base.
                _ = d.addResource(.irq, io.gsi_base, 0);
            },
            2 => {
                const iso: *const MadtIso = @ptrCast(base + off);
                if (platform_information.override_count < platform_information.overrides.len) {
                    platform_information.overrides[platform_information.override_count] = .{
                        .source = iso.source,
                        .gsi = iso.gsi,
                        .flags = iso.flags,
                    };
                    platform_information.override_count += 1;
                }
            },
            5 => {
                const ovr: *const MadtLapicOverride = @ptrCast(base + off);
                platform_information.lapic_base = ovr.address;
            },
            else => {},
        }
        off += rec.length;
    }
}

/// MCFG -> a pci_host_bridge per ECAM segment, then a PCI enumeration underneath.
fn parseMcfg(device_tree: *DeviceTree, header: *const SystemDescriptorTableHeader) !void {
    const total: usize = header.length;
    const base: [*]const u8 = @ptrCast(header);

    var off: usize = @sizeOf(Mcfg);
    while (off + @sizeOf(McfgAllocation) <= total) : (off += @sizeOf(McfgAllocation)) {
        const alloc: *const McfgAllocation = @ptrCast(base + off);
        const bus_count: u64 = @as(u64, alloc.end_bus - alloc.start_bus) + 1;

        var nb: [24]u8 = undefined;
        const nm = std.fmt.bufPrint(&nb, "pci{d}", .{alloc.segment_group}) catch "pci";
        const bridge = try device_tree.addChild(device_tree.root, .pci_host_bridge, nm);
        // ECAM window: 1 MiB of configuration space per bus.
        _ = bridge.addResource(.memory, alloc.base_address, bus_count << 20);
        _ = bridge.addResource(.bus_range, alloc.start_bus, bus_count);
        addBridgeApertures(bridge);
        // The bridge decodes the whole 16-bit I/O space toward its bus — the
        // window functions' I/O BARs must register-contain within (M19.2).
        _ = bridge.addResource(.io_port, 0, 1 << 16);

        // The function walk itself retired to ring 3 (M19.3): the pci-bus
        // driver claims this bridge, repeats the scan through its ECAM grant,
        // and device_registers what it finds — the kernel seeds only the
        // bridge. The scan's equivalence was proven before the hand-off
        // (pci-scan), and the walk's history is in git if archaeology calls.
    }
}

/// The boot memory map, stored at discover() entry for the aperture derivation
/// below (and, in M20, for the acpi-tables node's containment windows).
var boot_memory_regions: []const boot_handoff.MemoryRegion = &.{};

/// The bridge's MMIO apertures, derived from the boot memory map's holes
/// (docs/discovery.md — apertures from the memory map): registered PCI functions carry BAR
/// resources, and `device_register` containment demands the bridge own windows
/// that cover them. Everything the firmware described is "not hole"; the low
/// aperture runs from the end of the described space below 4 GiB up to the
/// I/O-APIC region, the high one from 4 GiB (or the end of RAM above it) to
/// the 46-bit line. Coarse, mechanical, and AML-free — available at boot no
/// matter what later moved to user space.
fn addBridgeApertures(bridge: *device_model.Device) void {
    // Below 4 GiB the described regions are sparse (RAM low, firmware flash
    // and tables high), so the holes are the *gaps between* them — a single
    // "after the last region" rule dies on OVMF's flash at the very top.
    // Sort-merge the described ranges, then keep the three largest gaps
    // (resource slots are bounded at 8 per device; ECAM + bus range + 3 + the
    // high aperture fits). Above 4 GiB one aperture runs from the end of the
    // described space to the 46-bit line.
    const Range = struct { base: u64, end: u64 };
    var below: [64]Range = undefined;
    var below_count: usize = 0;
    var high_end: u64 = 1 << 32;
    for (boot_memory_regions) |region| {
        const end = region.base + region.pages * 4096;
        // Above 4 GiB only *usable RAM* blocks the aperture: OVMF describes
        // its own 64-bit PCI window as a reserved region and then programs
        // BARs inside it — honoring reserved there would exclude the very
        // space BARs live in. Below 4 GiB every described region blocks (the
        // kernel image, the tables, the ramdisk all live there). Bring-up
        // trust: only the bridge's claimant can register into the aperture.
        if (region.kind == .usable and end > high_end) high_end = end;
        if (region.base >= (1 << 32) or below_count == below.len) continue;
        below[below_count] = .{ .base = region.base, .end = @min(end, 1 << 32) };
        below_count += 1;
    }
    // Insertion sort by base (the map is small and this runs once at boot).
    for (1..below_count) |i| {
        const key = below[i];
        var j = i;
        while (j > 0 and below[j - 1].base > key.base) : (j -= 1) below[j] = below[j - 1];
        below[j] = key;
    }
    // Walk the sorted ranges, collecting inter-region gaps of at least 1 MiB.
    var gaps: [3]Range = .{Range{ .base = 0, .end = 0 }} ** 3;
    var cursor: u64 = 0;
    var index: usize = 0;
    while (index <= below_count) : (index += 1) {
        const gap_end = if (index == below_count) (1 << 32) else below[index].base;
        if (gap_end > cursor and gap_end - cursor >= (1 << 20)) {
            // Keep the three largest, replacing the smallest kept so far.
            var smallest: usize = 0;
            for (gaps, 0..) |gap, gi| {
                if (gap.end - gap.base < gaps[smallest].end - gaps[smallest].base) smallest = gi;
            }
            if (gap_end - cursor > gaps[smallest].end - gaps[smallest].base) {
                gaps[smallest] = .{ .base = cursor, .end = gap_end };
            }
        }
        if (index < below_count and below[index].end > cursor) cursor = below[index].end;
    }
    for (gaps) |gap| {
        if (gap.end > gap.base) _ = bridge.addResource(.memory, gap.base, gap.end - gap.base);
    }
    _ = bridge.addResource(.memory, high_end, (@as(u64, 1) << 46) - high_end);
}

/// HPET -> a timer node with its register block as an MMIO resource, plus the GSI
/// its comparators can raise.
///
/// Unlike a PCI device or an ACPI `_CRS` node, the HPET table carries **no interrupt
/// number**: which I/O APIC inputs a comparator may drive is advertised at runtime,
/// as a bitmask in `Tn_INT_ROUTE_CAP` (bits 63:32 of the Timer 0 configuration register).
/// So discovery maps the register block, reads the mask, and records one concrete
/// `irq` resource — the GSI a driver is entitled to bind. The driver commits to it
/// by writing `Tn_INT_ROUTE_CNF`; the kernel checks the binding against this
/// resource (see process.ownedGsi), which is what keeps `irq_bind` a capability
/// rather than a request for an arbitrary interrupt line.
fn parseHpet(device_tree: *DeviceTree, hal: Hal, header: *const SystemDescriptorTableHeader) !void {
    const hpet: *const Hpet = @ptrCast(header);
    const d = try device_tree.addChild(device_tree.root, .timer, "hpet");

    // The GAS tag must say System Memory (0) before we treat `address` as a physical
    // address. The HPET spec mandates it, but firmware is not a thing to trust: a
    // System I/O (1) tag here would have us map an arbitrary page and read a bogus
    // route-capability mask out of it.
    if (hpet.address_space_id != gas_system_memory) return;

    _ = d.addResource(.memory, hpet.address, 0x400);

    const regs = hal.mapMmio(hpet.address, 0x400, true);
    const t0_configuration: *const volatile u64 = @ptrFromInt(regs + 0x100);
    const route_cap: u32 = @truncate(t0_configuration.* >> 32);
    if (hpetGsi(route_cap)) |gsi| _ = d.addResource(.irq, gsi, 1);
}

/// ACPI Generic Address Structure address-space ids we care about.
const gas_system_memory: u8 = 0;

/// Pick a GSI for the HPET out of its route-capability mask. Prefer an input at or
/// above 16: the low ones overlap the legacy ISA lines (2 = cascaded PIT, 8 = RTC),
/// which the MADT may separately override, whereas 16+ are the free upper inputs on
/// every I/O APIC we care about. Falls back to the lowest bit set if there are none.
fn hpetGsi(route_cap: u32) ?u32 {
    if (route_cap == 0) return null;
    var gsi: u32 = 16;
    while (gsi < 32) : (gsi += 1) {
        if (route_cap & (@as(u32, 1) << @intCast(gsi)) != 0) return gsi;
    }
    return @ctz(route_cap);
}

// FADT field offsets (bytes from the table start). The FADT grew across ACPI
// revisions, so every field is read through `fadt()` with a length guard rather
// than a fixed struct — an older/shorter FADT simply lacks the later (X_) fields.
const fadt_dsdt = 40; // u32
const fadt_smi_cmd = 48; // u32 (an I/O port)
const fadt_acpi_enable = 52; // u8
const fadt_acpi_disable = 53; // u8
const fadt_pm1a_cnt_blk = 64; // u32 (I/O port)
const fadt_pm1b_cnt_blk = 68; // u32 (I/O port)
const fadt_pm_tmr_blk = 76; // u32 (I/O port) — the PM timer counter
const fadt_pm1_cnt_len = 89; // u8 (bytes)
const fadt_sci_int = 46; // u16 (the SCI's GSI)
const fadt_flags = 112; // u32
const fadt_reset_register = 116; // GAS (12 bytes)
const fadt_reset_value = 128; // u8
const fadt_x_dsdt = 140; // u64
const fadt_x_pm1a_cnt_blk = 172; // GAS
const fadt_x_pm1b_cnt_blk = 184; // GAS
const fadt_x_pm_tmr_blk = 208; // GAS
const flag_reset_register_supported = 1 << 10;
const flag_tmr_value_ext = 1 << 8; // PM timer counter is 32-bit (else 24-bit)

/// FADT -> the power register map (into `power_information`) and the DSDT address,
/// whose bytecode is collected for the ring-3 parse. No AML interpretation here.
fn parseFadt(header: *const SystemDescriptorTableHeader) void {
    const base: [*]align(1) const u8 = @ptrCast(header);
    const len: usize = header.length;
    const pi = &power_information;

    pi.sci_interrupt = @truncate(fadt(u16, base, len, fadt_sci_int) orelse 0);
    pi.smi_cmd = @truncate(fadt(u32, base, len, fadt_smi_cmd) orelse 0);
    pi.acpi_enable = fadt(u8, base, len, fadt_acpi_enable) orelse 0;
    pi.acpi_disable = fadt(u8, base, len, fadt_acpi_disable) orelse 0;

    const cnt_width = fadt(u8, base, len, fadt_pm1_cnt_len) orelse 2;
    pi.pm1a_cnt = readCntRegister(base, len, fadt_x_pm1a_cnt_blk, fadt_pm1a_cnt_blk, cnt_width);
    pi.pm1b_cnt = readCntRegister(base, len, fadt_x_pm1b_cnt_blk, fadt_pm1b_cnt_blk, cnt_width);

    const flags = fadt(u32, base, len, fadt_flags) orelse 0;
    pi.reset_supported = flags & flag_reset_register_supported != 0;
    pi.reset = readGas(base, len, fadt_reset_register) orelse .{};
    pi.reset_value = fadt(u8, base, len, fadt_reset_value) orelse 0;

    // The PM timer — a fixed-rate counter used as a calibration reference when no
    // HPET is present. Prefer the 64-bit-capable X_ GAS, fall back to the port.
    platform_information.pm_timer = readCntRegister(base, len, fadt_x_pm_tmr_blk, fadt_pm_tmr_blk, 4);
    platform_information.pm_timer_32bit = flags & flag_tmr_value_ext != 0;

    var dsdt: u64 = fadt(u32, base, len, fadt_dsdt) orelse 0;
    if (fadt(u64, base, len, fadt_x_dsdt)) |x| {
        if (x != 0) dsdt = x;
    }
    dsdt_physical = dsdt;
    addAmlBlock(dsdt);
}

// SPCR field offsets (bytes from the table start).
const spcr_interface_type = 36; // u8
const spcr_base_address = 40; // GAS (12 bytes)

/// SPCR -> the console UART's address + interface type, so serial can target the
/// firmware's actual debug port instead of assuming legacy COM1.
fn parseSpcr(header: *const SystemDescriptorTableHeader) void {
    const base: [*]align(1) const u8 = @ptrCast(header);
    const len: usize = header.length;
    const gas = readGas(base, len, spcr_base_address) orelse return;
    if (gas.address == 0) return;
    platform_information.spcr_uart = gas;
    platform_information.spcr_kind = fadt(u8, base, len, spcr_interface_type) orelse 0;
}

// DMAR remapping-structure layout (Intel VT-d spec §8): the DMAR-specific header is 12
// bytes (host-address-width, flags, 10 reserved), then a list of {type u16, length u16}
// structures. Type 0 is a DRHD (DMA Remapping Hardware Unit Definition): flags byte at
// offset 4 (bit 0 = INCLUDE_PCI_ALL, the catch-all unit), 64-bit register base at
// offset 8. Type 1 is an RMRR (Reserved Memory Region Reporting): a physical range at
// offsets 8/16 (base / inclusive limit) that the device(s) named by the trailing
// device-scope entries keep DMAing into across the firmware→OS handoff.
const dmar_structures_offset = 48; // 36-byte ACPI header + 12-byte DMAR header
const dmar_type_drhd: u16 = 0;
const dmar_type_rmrr: u16 = 1;
const drhd_flags_offset = 4;
const drhd_include_pci_all: u8 = 1;
const drhd_register_base_offset = 8;
const rmrr_base_offset = 8;
const rmrr_limit_offset = 16;
const rmrr_scopes_offset = 24;
// Device-scope entry (within DRHD/RMRR structures): type 1 = PCI endpoint; the path is
// (device, function) byte pairs from offset 6, one pair per bridge hop plus the leaf.
const scope_type_pci_endpoint: u8 = 1;
const scope_start_bus_offset = 5;
const scope_path_offset = 6;

/// DMAR -> the VT-d unit(s) and reserved memory regions. Walks every remapping
/// structure: selects the INCLUDE_PCI_ALL DRHD (the catch-all covering all devices not
/// scoped elsewhere — commonly the SECOND unit on real machines, after an iGPU-scoped
/// one), counts the rest so the boot log can warn that their devices stay untranslated,
/// and records single-path endpoint RMRRs for the IOMMU core to pre-map before it
/// enables translation. Multi-hop RMRR scopes are skipped loudly: better a named gap
/// than a silent one.
fn parseDmar(header: *const SystemDescriptorTableHeader) void {
    const base: [*]align(1) const u8 = @ptrCast(header);
    const total: usize = header.length;

    var off: usize = dmar_structures_offset;
    while (off + 4 <= total) {
        const kind = fadt(u16, base, total, off) orelse break;
        const length = fadt(u16, base, total, off + 2) orelse break;
        if (length < 4 or off + length > total) break; // malformed; stop rather than loop
        if (kind == dmar_type_drhd) {
            const register_base = fadt(u64, base, total, off + drhd_register_base_offset) orelse 0;
            const include_all = ((fadt(u8, base, total, off + drhd_flags_offset) orelse 0) & drhd_include_pci_all) != 0;
            if (register_base != 0) {
                // Selection: the INCLUDE_PCI_ALL unit wins; otherwise keep the first
                // seen. A later catch-all replaces an earlier scoped unit.
                const replace = !platform_information.iommu_present or
                    (include_all and !platform_information.iommu_include_all);
                if (replace) {
                    // Table facts only: the unit's registers are the IOMMU
                    // backend's business (it maps and validates them at
                    // detect) — discovery records where they live, never
                    // reads them.
                    if (platform_information.iommu_present) platform_information.iommu_extra_units += 1;
                    platform_information.iommu_present = true;
                    platform_information.iommu_base = register_base;
                    platform_information.iommu_include_all = include_all;
                } else {
                    platform_information.iommu_extra_units += 1;
                }
            }
        } else if (kind == dmar_type_rmrr) {
            parseRmrr(base, total, off, length);
        }
        off += length;
    }
}

/// One RMRR structure: record a {bdf, base, limit} per single-path endpoint scope.
fn parseRmrr(base: [*]align(1) const u8, total: usize, off: usize, length: usize) void {
    const range_base = fadt(u64, base, total, off + rmrr_base_offset) orelse return;
    const range_limit = fadt(u64, base, total, off + rmrr_limit_offset) orelse return;
    if (range_limit < range_base) return;

    var scope = off + rmrr_scopes_offset;
    const end = off + length;
    while (scope + 6 <= end) {
        const scope_type = fadt(u8, base, total, scope) orelse break;
        const scope_length = fadt(u8, base, total, scope + 1) orelse break;
        if (scope_length < 6 or scope + scope_length > end) break;
        if (scope_type == scope_type_pci_endpoint and scope_length == scope_path_offset + 2) {
            // Single (device, function) pair: a directly-reachable endpoint.
            const bus = fadt(u8, base, total, scope + scope_start_bus_offset) orelse 0;
            const device = fadt(u8, base, total, scope + scope_path_offset) orelse 0;
            const function = fadt(u8, base, total, scope + scope_path_offset + 1) orelse 0;
            if (platform_information.rmrr_count < maximum_rmrr) {
                platform_information.rmrr[platform_information.rmrr_count] = .{
                    .bdf = (@as(u16, bus) << 8) | (@as(u16, device) << 3) | function,
                    .base = range_base,
                    .limit = range_limit,
                };
                platform_information.rmrr_count += 1;
            } else {
                platform_information.rmrr_skipped +|= 1; // table full
            }
        } else {
            platform_information.rmrr_skipped +|= 1; // multi-hop path or non-endpoint scope
        }
        scope += scope_length;
    }
}

// IVRS layout (AMD I/O Virtualization spec): 36-byte ACPI header, IVinfo u32 @36,
// 8 reserved @40, then IVHD/IVMD blocks from @48. An IVHD common header is type u8 @0,
// flags u8 @1, length u16 @2, device id u16 @4, capability offset u16 @6, IOMMU base
// address u64 @8, PCI segment u16 @16, IOMMU info u16 @18.
const ivrs_blocks_offset = 48;
const ivhd_type_10: u8 = 0x10;
const ivhd_type_11: u8 = 0x11;
const ivhd_base_offset = 8;

/// IVRS -> detect an AMD-Vi IOMMU. Record the control-register base from the first IVHD
/// of type 0x10/0x11. Per-device entries and IVMD (the AMD analogue of RMRR) are ignored
/// in v1 — the default-deny device table is what we build anyway, and QEMU emits no IVMD;
/// a real machine that needs them is flagged untested on AMD regardless.
fn parseIvrs(header: *const SystemDescriptorTableHeader) void {
    const base: [*]align(1) const u8 = @ptrCast(header);
    const total: usize = header.length;
    var off: usize = ivrs_blocks_offset;
    while (off + 4 <= total) {
        const kind = fadt(u8, base, total, off) orelse break;
        const length = fadt(u16, base, total, off + 2) orelse break;
        if (length < 4 or off + length > total) break;
        if (kind == ivhd_type_10 or kind == ivhd_type_11) {
            const iommu_base = fadt(u64, base, total, off + ivhd_base_offset) orelse 0;
            if (iommu_base != 0) {
                platform_information.iommu_present = true;
                platform_information.iommu_is_amd = true;
                platform_information.iommu_base = iommu_base;
                return; // first IVHD is enough; multi-unit is future work
            }
        }
        off += length;
    }
}

// --- helpers ----------------------------------------------------------------

/// Sum `len` bytes; an ACPI table/pointer is valid when the low 8 bits are zero.
fn checksumOk(bytes: [*]const u8, len: usize) bool {
    var sum: u8 = 0;
    for (0..len) |i| sum +%= bytes[i];
    return sum == 0;
}

/// Read a FADT field of type `T` at `off`, or null if the table is too short to
/// contain it (a legal state for older FADT revisions).
fn fadt(comptime T: type, base: [*]align(1) const u8, len: usize, off: usize) ?T {
    if (off + @sizeOf(T) > len) return null;
    return rd(T, base, off);
}

/// Decode a Generic Address Structure at `off` into a `RegisterAccess`. GAS layout:
/// address_space(u8), bit_width(u8), bit_offset(u8), access_size(u8), address(u64).
fn readGas(base: [*]align(1) const u8, len: usize, off: usize) ?RegisterAccess {
    if (off + 12 > len) return null;
    const address_space = rd(u8, base, off);
    const bit_width = rd(u8, base, off + 1);
    const address = rd(u64, base, off + 4);
    return .{
        .mmio = address_space == 0, // 0 = system memory, 1 = system I/O
        .address = address,
        .width = bit_width / 8,
    };
}

/// A PM1 control register: prefer the 64-bit-capable X_ GAS form; fall back to the
/// legacy 32-bit I/O-port field. Width comes from PM1_CNT_LEN either way.
fn readCntRegister(base: [*]align(1) const u8, len: usize, xoff: usize, legacy_off: usize, width: u8) RegisterAccess {
    if (readGas(base, len, xoff)) |g| {
        if (g.address != 0) return .{ .mmio = g.mmio, .address = g.address, .width = width };
    }
    const port = fadt(u32, base, len, legacy_off) orelse 0;
    return .{ .mmio = false, .address = port, .width = width };
}

/// Read a little-endian integer at `off` from a (possibly unaligned) byte pointer.
/// x86 is little-endian and native, so an unaligned load suffices.
fn rd(comptime T: type, bytes: [*]align(1) const u8, off: usize) T {
    const p: *align(1) const T = @ptrCast(bytes + off);
    return p.*;
}

// --- machine reboot -------------------------------------------------------------
// Restart via the FADT reset register (from `power_information` above), with legacy
// fallbacks, driven through the injected `Hal`. Soft-off (ACPI S5) and suspend (S3)
// are NOT here: they need the AML sleep-state (`_Sx`) values, which the kernel no
// longer parses — the ring-3 acpi service owns power management (it re-parses the
// blobs and writes the PM1 control register itself). Reboot stays in the kernel
// because it needs no AML — only the FADT reset register and the well-known legacy
// fallbacks — so it survives as a last-resort restart. See docs/power.md.

/// Restart the machine. Tries the ACPI reset register first, then the two legacy
/// fallbacks. Returns only if every method failed (very unlikely).
pub fn reboot(hal: device_model.Hal) void {
    const pi = power_information;

    // 1. The FADT reset register, when the firmware advertises support.
    if (pi.reset_supported and pi.reset.present()) {
        writeResetRegister(hal, pi.reset, pi.reset_value);
        rebootDelay();
    }
    // 2. The PCI reset-control register at port 0xCF9 (RST_CPU | SYSTEM_RST).
    hal.pioWrite(1, 0xCF9, 0x0E);
    hal.pioWrite(1, 0xCF9, 0x06);
    rebootDelay();
    // 3. Pulse the 8042 keyboard controller's reset line.
    hal.pioWrite(1, 0x64, 0xFE);
    rebootDelay();
}

fn writeResetRegister(hal: device_model.Hal, register: RegisterAccess, value: u32) void {
    if (register.mmio) {
        const p: *align(1) volatile u32 = @ptrFromInt(hal.mapMmio(register.address, 4, true));
        p.* = value;
    } else {
        hal.pioWrite(register.width, @intCast(register.address), value);
    }
}

/// A short busy-wait so a reset takes effect before we fall through to the next
/// method. The empty asm is an architecture-neutral barrier that keeps the loop
/// from being optimised away.
fn rebootDelay() void {
    var i: usize = 0;
    while (i < 50_000_000) : (i += 1) {
        asm volatile ("" ::: .{ .memory = true });
    }
}