// SPDX-License-Identifier: MIT OR Apache-2.0
//
// Copyright (c) 2021-2022 Andre Richter <andre.o.richter@gmail.com>

//! Architectural boot code.
//!
//! # Orientation
//!
//! Since arch modules are imported into generic modules using the path attribute, the path of this
//! file is:
//!
//! crate::cpu::boot::arch_boot

use aarch64_cpu::{asm, registers::*};
use core::arch::global_asm;
use tock_registers::interfaces::Writeable;

// Assembly counterpart to this file.
global_asm!(
    include_str!("boot.s"),
    CONST_CURRENTEL_EL2 = const 0x8,
    CONST_CORE_ID_MASK = const 0b11
);

//--------------------------------------------------------------------------------------------------
// Private Code
//--------------------------------------------------------------------------------------------------

/// Prepares the transition from EL2 to EL1.
///
/// # Safety
///
/// - The `bss` section is not initialized yet. The code must not use or reference it in any way.
/// - The HW state of EL1 must be prepared in a sound way.
#[inline(always)]
unsafe fn prepare_el2_to_el1_transition(phys_boot_core_stack_end_exclusive_addr: u64) {
    // Enable timer counter registers for EL1.
    CNTHCTL_EL2.write(CNTHCTL_EL2::EL1PCEN::SET + CNTHCTL_EL2::EL1PCTEN::SET);

    // No offset for reading the counters.
    CNTVOFF_EL2.set(0);

    // Set EL1 execution state to AArch64.
    HCR_EL2.write(HCR_EL2::RW::EL1IsAarch64);

    // Set up a simulated exception return.
    //
    // First, fake a saved program status where all interrupts were masked and SP_EL1 was used as a
    // stack pointer.
    SPSR_EL2.write(
        SPSR_EL2::D::Masked
            + SPSR_EL2::A::Masked
            + SPSR_EL2::I::Masked
            + SPSR_EL2::F::Masked
            + SPSR_EL2::M::EL1h,
    );

    // Second, let the link register point to kernel_init().
    ELR_EL2.set(crate::kernel_init as *const () as u64);

    // Set up SP_EL1 (stack pointer), which will be used by EL1 once we "return" to it. Since there
    // are no plans to ever return to EL2, just re-use the same stack.
    SP_EL1.set(phys_boot_core_stack_end_exclusive_addr);
}

//--------------------------------------------------------------------------------------------------
// Public Code
//--------------------------------------------------------------------------------------------------

/// The Rust entry of the `kernel` binary.
///
/// The function is called from the assembly `_start` function.
///
/// # Safety
///
/// - Exception return from EL2 must must continue execution in EL1 with `kernel_init()`.
#[no_mangle]
pub unsafe extern "C" fn _start_rust(phys_boot_core_stack_end_exclusive_addr: u64) -> ! {
    prepare_el2_to_el1_transition(phys_boot_core_stack_end_exclusive_addr);

    // Use `eret` to "return" to EL1. This results in execution of kernel_init() in EL1.
    asm::eret()
}