et_kernel/

lib.rs

//! Shared `no_std` helpers for ET-SoC-1 compute kernels: hart identity, the
//! U-mode trace write, a hardware memory fence, and scratchpad addressing.
//!
//! This is the device-side support library for the compute kernels in this
//! package. Each kernel binary provides its own panic handler and invokes
//! [`kernel_entry!`] to generate the `_start` entry point.

#![no_std]

use core::arch::asm;
use core::ptr::{read_volatile, write_volatile};

/// Generate the kernel entry point (`_start`).
///
/// Expands to the naked `_start` every ET-SoC-1 kernel needs: placed in
/// `.text.init` (which the linker script lays down first at the fixed U-mode
/// entry address), it sets the global pointer, calls the kernel's `entry_point`,
/// and returns to firmware via `ecall`. The launch-args pointer arrives in `a0`
/// and passes straight through to `entry_point`'s first argument.
///
/// The kernel binary must define
/// `#[unsafe(no_mangle)] pub extern "C" fn entry_point(args_ptr: usize) -> i64`.
/// Invoke this once at the crate root:
///
/// ```ignore
/// et_kernel::kernel_entry!();
/// ```
#[macro_export]
macro_rules! kernel_entry {
    () => {
        #[unsafe(naked)]
        #[unsafe(no_mangle)]
        #[unsafe(link_section = ".text.init")]
        pub extern "C" fn _start() -> ! {
            ::core::arch::naked_asm!(
                ".option push",
                ".option norelax",
                "la gp, __global_pointer$",
                ".option pop",
                "call entry_point",
                "li a2, 0",  // KERNEL_RETURN_SUCCESS
                "mv a1, a0", // return value
                "li a0, 8",  // SYSCALL_RETURN_FROM_KERNEL
                "ecall",
            )
        }
    };
}

/// Base of the per-hart U-mode trace control-block array
/// (`CM_UMODE_TRACE_CB_BASEADDR`); each entry is 64 bytes.
pub const CB_BASE: usize = 0x8004_F23000;
const CB_STRIDE: usize = 64;
const CB_BASE_PER_HART: usize = 24;
const CB_OFFSET_PER_HART: usize = 36;
const TRACE_TYPE_STRING: u16 = 0;
const ENTRY_HEADER_SIZE: usize = 16;
const TRACE_STRING_MAX: usize = 512;

/// Current hart ID, from the custom `hartid` CSR (`0xCD0`).
#[inline(always)]
pub fn hart_id() -> u32 {
    let v: u64;
    // SAFETY: reads a U-mode-accessible CSR with no side effects.
    unsafe { asm!("csrr {0}, 0xcd0", out(reg) v, options(nomem, nostack, preserves_flags)) };
    v as u32
}

/// Current shire ID (`hart_id >> 6`; 64 harts per shire).
#[inline(always)]
pub fn shire_id() -> u32 {
    hart_id() >> 6
}

/// A cycle timestamp (`hpmcounter3`, CSR `0xC03`) for trace entry headers.
///
/// Applies the RTLMIN-6496 workaround: four back-to-back reads of CSR `0xC03`
/// in a 16-byte-aligned block; the fourth read is the reliable value.
#[inline(always)]
pub fn timestamp() -> u64 {
    let v: u64;
    // SAFETY: reads a U-mode-accessible performance-counter CSR with no
    // side effects. Four reads are required by the RTLMIN-6496 erratum;
    // a single asm block prevents the compiler inserting intervening code.
    // `nomem` is omitted so the block is treated as a memory barrier.
    unsafe {
        asm!(
            ".align 4",
            "csrrs {v0}, 0xC03, x0",
            "csrrs {v1}, 0xC03, x0",
            "csrrs {v2}, 0xC03, x0",
            "csrrs {v},  0xC03, x0",
            v0 = out(reg) _,
            v1 = out(reg) _,
            v2 = out(reg) _,
            v  = out(reg) v,
            options(nostack, preserves_flags),
        )
    };
    v
}

/// Full hardware memory fence (`fence rw, rw`) that also bars compiler
/// reordering. This is an ordering barrier, not an atomic operation.
#[inline(always)]
pub fn fence() {
    // No `nomem`: the asm is treated as touching memory, so the compiler will
    // not move loads/stores across it either.
    unsafe { asm!("fence rw, rw", options(nostack, preserves_flags)) };
}

/// Base address of `shire`'s 2.5 MB L2 scratchpad
/// (`ETSOC_SCP_GET_SHIRE_ADDR(shire, 0)`): `0x8000_0000 | (shire << 23)`.
#[inline(always)]
pub fn scp_shire_base(shire: u32) -> usize {
    0x8000_0000usize + ((shire as usize) << 23)
}

#[inline(always)]
fn cb_index(hart: u32) -> usize {
    if hart < 2048 {
        hart as usize
    } else {
        (hart - 32) as usize
    }
}

#[inline(always)]
fn align8(n: usize) -> usize {
    (n + 7) & !7
}

/// Write `text` as a NUL-terminated string trace entry for the current hart,
/// exactly as the SDK's `Trace_String` does (reserve via the control block, then
/// write a `trace_string_t`).
pub fn trace_str(text: &[u8]) {
    let hid = hart_id();
    let str_len = align8(text.len() + 1).min(TRACE_STRING_MAX);
    let cb = CB_BASE + cb_index(hid) * CB_STRIDE;
    // SAFETY: firmware populated the CB at this fixed address before launch.
    let base = unsafe { read_volatile((cb + CB_BASE_PER_HART) as *const u64) } as usize;
    let offset = unsafe { read_volatile((cb + CB_OFFSET_PER_HART) as *const u32) };
    let head = base + offset as usize;
    // SAFETY: `head` lies within this hart's reserved trace-buffer slice.
    unsafe {
        write_volatile(head as *mut u64, timestamp());
        write_volatile((head + 8) as *mut u32, str_len as u32);
        write_volatile((head + 12) as *mut u16, hid as u16);
        write_volatile((head + 14) as *mut u16, TRACE_TYPE_STRING);
        let s = (head + ENTRY_HEADER_SIZE) as *mut u8;
        let mut i = 0;
        while i < str_len {
            let byte = if i < text.len() { text[i] } else { 0 };
            write_volatile(s.add(i), byte);
            i += 1;
        }
        write_volatile(
            (cb + CB_OFFSET_PER_HART) as *mut u32,
            offset + (ENTRY_HEADER_SIZE + str_len) as u32,
        );
    }
}

/// A fixed-capacity stack buffer for composing trace messages without `alloc`.
pub struct MsgBuf {
    buf: [u8; 192],
    len: usize,
}

impl Default for MsgBuf {
    fn default() -> Self {
        Self::new()
    }
}

impl MsgBuf {
    pub fn new() -> Self {
        MsgBuf {
            buf: [0; 192],
            len: 0,
        }
    }

    /// Append raw text (truncated if the buffer fills).
    pub fn str(&mut self, s: &[u8]) -> &mut Self {
        let mut i = 0;
        while i < s.len() && self.len < self.buf.len() {
            self.buf[self.len] = s[i];
            self.len += 1;
            i += 1;
        }
        self
    }

    /// Append a decimal integer.
    pub fn u64(&mut self, mut v: u64) -> &mut Self {
        let mut tmp = [0u8; 20];
        let mut c = 0;
        loop {
            tmp[c] = b'0' + (v % 10) as u8;
            v /= 10;
            c += 1;
            if v == 0 {
                break;
            }
        }
        while c > 0 && self.len < self.buf.len() {
            c -= 1;
            self.buf[self.len] = tmp[c];
            self.len += 1;
        }
        self
    }

    pub fn as_slice(&self) -> &[u8] {
        &self.buf[..self.len]
    }
}

/// Cache-line size in bytes. Per-hart outputs are placed one-per-line so that
/// distinct harts never write the same line: false sharing silently corrupts
/// data on this software-coherent architecture. Defined once in `et-abi` and
/// shared with the host, so the two sides cannot disagree on the stride.
pub use et_abi::CACHE_LINE;

/// A hart's view of an SPMD launch: its identity within `n_harts` participants.
///
/// The safety story of the reduction demo lives here. A kernel body, given a
/// `Grid`, can obtain only *its own* disjoint slice of the input and *its own*
/// output cell -- it has no way to name another hart's data, so cross-hart data
/// races are unrepresentable in the (safe) kernel body. The small `unsafe`
/// boundary that turns device addresses into slices is confined to this module.
pub struct Grid {
    hart: u32,
    n_harts: u32,
}

impl Grid {
    /// Build from the current hart's id and the number of participating harts.
    pub fn new(n_harts: u32) -> Self {
        Grid {
            hart: hart_id(),
            n_harts,
        }
    }

    pub fn hart(&self) -> u32 {
        self.hart
    }

    pub fn n_harts(&self) -> u32 {
        self.n_harts
    }

    /// Whether this hart participates (the launch runs every hart of the shire,
    /// so surplus harts opt out).
    pub fn active(&self) -> bool {
        self.hart < self.n_harts
    }

    /// This hart's half-open element range of a length-`n` domain: contiguous,
    /// disjoint across harts, and together covering all of `[0, n)` (a balanced
    /// split, the first `n % n_harts` harts taking one extra element).
    fn range(&self, n: usize) -> (usize, usize) {
        let h = self.hart as usize;
        let p = (self.n_harts as usize).max(1);
        let base = n / p;
        let rem = n % p;
        let start = h * base + h.min(rem);
        let len = base + if h < rem { 1 } else { 0 };
        (start, start + len)
    }

    /// Borrow this hart's disjoint sub-slice of `data`.
    pub fn my_slice<'a, T>(&self, data: &'a [T]) -> &'a [T] {
        let (start, end) = self.range(data.len());
        &data[start..end]
    }

    /// Borrow this hart's own output cell from an array of one cache-line-padded
    /// `T` per hart based at device address `base`.
    ///
    /// # Safety
    /// `base` must address at least `n_harts * CACHE_LINE` writable bytes of
    /// device memory. Disjointness across harts is guaranteed by construction
    /// (distinct `hart` ids map to distinct cache lines).
    pub unsafe fn output_cell<'a, T>(&self, base: usize) -> &'a mut T {
        unsafe { &mut *((base + self.hart as usize * CACHE_LINE) as *mut T) }
    }
}

/// Tensor-extension intrinsics: TensorLoad, TensorLoadB, TensorFMA32,
/// TensorStore, and TensorWait, all encoded as RISC-V CSR writes.
pub mod tensor;

/// Performance Monitoring Unit (PMU) counter API.
///
/// Provides [`pmu::pmu_read`] (reads `hpmcounterN` in U-mode) and the
/// [`pmu::PmuEvent`] event-code enum for characterising tensor kernel behaviour.
pub mod pmu;

/// Packed-single (PS) SIMD intrinsics for 256-bit FP registers.
///
/// All functions are stubs pending confirmation of the PS opcode encodings
/// from PRM Chapter 5. This module is hidden from published documentation
/// until the implementations are verified on hardware.
#[doc(hidden)]
pub mod simd;

/// View `n` elements of type `T` at device address `addr` as a shared slice.
///
/// # Safety
/// `addr` must point to `n` valid, aligned, initialised `T` that outlive the
/// returned borrow and are not mutated through another path meanwhile.
pub unsafe fn device_slice<'a, T>(addr: usize, n: usize) -> &'a [T] {
    unsafe { core::slice::from_raw_parts(addr as *const T, n) }
}