danos/system/kernel/console.zig

//! A framebuffer text console: draws glyphs from an embedded PSF2 font directly
//! into the linear framebuffer the bootloader handed us. No firmware, no driver
//! — just pixels.
//!
//! This is a **bootstrap** console — a stop-gap so early boot has something on
//! screen. The framebuffer is a general graphics surface, *not* inherently a text
//! terminal; once the driver machinery exists it becomes a proper graphics device
//! driver and this text-grid crutch goes away. It is therefore kept **separate
//! from the diagnostic [log](log.zig)** — the log fans out to serial/debugcon/file,
//! while this only paints the handful of user-facing status lines and panics.
//!
//! The module owns a single console and a `present` flag; `write` is a no-op when
//! the firmware handed over no framebuffer (a headless machine), so the kernel
//! never assumes a display exists.

const std = @import("std");
const boot_handoff = @import("boot-handoff");

/// The one framebuffer console, valid only when `con_present`.
var con: Console = undefined;
var con_present: bool = false;

/// Set up the console over `fb`, or mark it absent if there's no usable
/// framebuffer. Clears the screen when present.
pub fn init(fb: boot_handoff.Framebuffer) void {
    if (!fb.present()) {
        con_present = false;
        return;
    }
    con = Console.init(fb);
    if (con.cols == 0 or con.rows == 0) {
        con_present = false;
        return;
    }
    con.clear();
    con_present = true;
}

/// Whether an on-screen console is available.
pub fn present() bool {
    return con_present;
}

/// Output sink: draw `bytes` on screen. A no-op when no framebuffer is present,
/// so it's always safe to call.
pub fn write(bytes: []const u8) void {
    if (!con_present) return;
    for (bytes) |c| con.putChar(c);
}

/// The console font, embedded at compile time. cp850-8x16, PSF2 format:
/// a 32-byte header, then 256 glyphs of 16 bytes each (one byte per 8-pixel
/// row). We index glyphs straight by byte value, so ASCII maps 1:1.
const font = @embedFile("font.psf");
const glyph_w = 8;
const glyph_h = 16;
const glyph_bytes = glyph_h; // 8 pixels wide => 1 byte per row
const glyph_data = 32; // PSF2 header size

pub const Console = struct {
    fb: boot_handoff.Framebuffer,
    cols: u32,
    rows: u32,
    col: u32 = 0,
    row: u32 = 0,
    fg: u32 = 0x00c8_c8c8, // light grey
    bg: u32 = 0x0000_0000, // black

    pub fn init(fb: boot_handoff.Framebuffer) Console {
        // Reach the framebuffer through the physmap, so the pointer stays valid
        // once the low identity map is gone. The base is mapped by both the
        // loader's bootstrap tables and paging.init.
        var mapped = fb;
        if (fb.base != 0) mapped.base = boot_handoff.physicalToVirtual(fb.base);
        return .{
            .fb = mapped,
            .cols = fb.width / glyph_w,
            .rows = fb.height / glyph_h,
        };
    }

    /// Fill the whole screen with the background colour and home the cursor.
    pub fn clear(self: *Console) void {
        var y: u32 = 0;
        while (y < self.fb.height) : (y += 1) self.fillRow(y, self.bg);
        self.col = 0;
        self.row = 0;
    }

    pub fn putChar(self: *Console, ch: u8) void {
        switch (ch) {
            '\n' => self.newline(),
            '\r' => self.col = 0,
            else => {
                if (self.col >= self.cols) self.newline();
                self.drawGlyph(ch, self.col * glyph_w, self.row * glyph_h);
                self.col += 1;
            },
        }
    }

    fn newline(self: *Console) void {
        self.col = 0;
        if (self.row + 1 >= self.rows) {
            self.scroll();
        } else {
            self.row += 1;
        }
    }

    fn drawGlyph(self: *Console, ch: u8, px: u32, py: u32) void {
        const rows = font[glyph_data + @as(usize, ch) * glyph_bytes ..][0..glyph_bytes];
        var gy: u32 = 0;
        while (gy < glyph_h) : (gy += 1) {
            const bits = rows[gy];
            var gx: u32 = 0;
            while (gx < glyph_w) : (gx += 1) {
                // Leftmost pixel is the high bit.
                const on = (bits >> @as(u3, @intCast(7 - gx))) & 1 != 0;
                self.pixel(px + gx, py + gy, if (on) self.fg else self.bg);
            }
        }
    }

    /// Shift the visible text up one glyph row and clear the freed bottom row,
    /// leaving the cursor on that now-blank last line.
    fn scroll(self: *Console) void {
        const visible = self.rows * glyph_h;
        var y: u32 = 0;
        while (y + glyph_h < visible) : (y += 1) self.copyRow(y, y + glyph_h);
        while (y < visible) : (y += 1) self.fillRow(y, self.bg);
        self.row = self.rows - 1;
    }

    inline fn rowPtr(self: *Console, y: u32) [*]volatile u32 {
        const base: [*]volatile u8 = @ptrFromInt(self.fb.base);
        return @ptrCast(@alignCast(base + y * self.fb.pitch));
    }

    inline fn pixel(self: *Console, x: u32, y: u32, color: u32) void {
        self.rowPtr(y)[x] = color;
    }

    fn fillRow(self: *Console, y: u32, color: u32) void {
        const row = self.rowPtr(y);
        var x: u32 = 0;
        while (x < self.fb.width) : (x += 1) row[x] = color;
    }

    fn copyRow(self: *Console, destination_y: u32, source_y: u32) void {
        const destination = self.rowPtr(destination_y);
        const source = self.rowPtr(source_y);
        var x: u32 = 0;
        while (x < self.fb.width) : (x += 1) destination[x] = source[x];
    }
};