//! Kernel tunables — the compile-time knobs, gathered in one place.
//!
//! These constants would otherwise be scattered across the files that use them,
//! hiding the trade-offs. Keeping them here makes them visible at a glance and gives
//! one spot to change them. They're plain `comptime` constants (zero runtime cost);
//! any one can later be promoted to a `-D` build option if a target needs to vary it
//! (see build.zig's `-Dtest-case` for the pattern). This keeps [[boot-handoff]] to what
//! it actually is — the loader↔kernel handoff *contract* — with tunables living here.
//!
//! **Kernel only, and deliberately so.** This file exists because tunables were
//! crowding the loader↔kernel contract they were split out of; it is not a registry for
//! the whole system. A driver's ring size belongs to that driver, a protocol's payload
//! cap to that protocol. How a ceiling is *declared*, wherever it lives, is
//! docs/os-development/bounds.md — a shape, not a shared list.

/// Ceiling on logical CPUs the kernel tracks — the size of the per-CPU bookkeeping
/// arrays (discovery pool, scheduler state, per-core GDT/TSS). Generous headroom:
/// those structs are small, and the *large* per-core resources (kernel and IST
/// stacks) are allocated at bring-up for cores that actually come online, so this
/// ceiling is cheap.
///
/// bound: logical CPUs the kernel tracks
/// decided-by: hardware
/// protects: the per-CPU bookkeeping arrays, which are sized at compile time
/// at-limit: degrade — the surplus cores are left parked, never brought online
/// observed-by: platform.cpusDropped() -> the WARNING at kernel.zig:281
pub const maximum_cpus = 128;

/// Maximum tasks (kernel threads) alive at once — the static task-table size. Each
/// online core consumes one slot for its idle task, plus task 0 on the BSP. Sized
/// for the initial-ramdisk sweep (the bundled binaries spawned at once) plus the
/// device manager's supervised children with room to grow — at 16 the sweep
/// started failing spawns once the bundle passed a dozen binaries. Raised to 48
/// for the USB stack: the xHCI bus driver spawns a supervised class-driver instance
/// per matched interface (keyboard, mouse, mass storage), on top of the FAT and
/// block servers and the growing ramdisk bundle.
///
/// The history above is the argument against this number: it has been raised twice,
/// each time by a machine or a bundle that outgrew it, which is the pattern the
/// bounds rule exists to stop. It is `ours` only because the task table is static;
/// how many drivers a machine needs is decided by how much hardware it has.
///
/// bound: kernel threads alive at once — the static task-table size
/// decided-by: hardware
/// protects: the statically allocated task table
/// at-limit: refuse — spawn fails; a supervised driver is never started
/// observed-by: the spawning supervisor's own log line; see docs/bounds-track-plan.md
pub const maximum_tasks = 48;

/// Each task's kernel stack (also each AP's bring-up stack), in bytes.
pub const kernel_stack_size = 16 * 1024;

/// Each user process's stack, in pages (32 KiB). Mapped just below a fixed top;
/// the System V entry block (argc/argv) occupies the top of the highest page, and
/// the page below the mapping is left unmapped as a guard, so an overflow faults
/// (killing only that process) instead of silently corrupting the image.
pub const user_stack_pages = 8;

/// Each core's IST (double-fault) stack, in bytes. The BSP's is static; an AP's is
/// heap-allocated at bring-up.
pub const ist_stack_size = 16 * 1024;

/// Scheduler tick / preemption rate, in Hz (the timer's periodic frequency).
pub const timer_hz = 1000;