// block_dev: block-device abstraction.
//
// The FAT32 backend talks to this; the board layer
// (src/board/<board>/emmc2.zig) populates the vtable post-init.
// A single global instance (`sd_dev`) covers the current "exactly one
// SD card" assumption — future work generalises if/when a second
// block device shows up (USB mass storage, virtio-blk, etc.).
//
// `extern struct` + `callconv(.c)` keep the layout reachable from
// assembly + future board ports without depending on the Zig
// compiler's internal calling convention.

pub const BlockDev = extern struct {
    read_fn ?*fn(u32, *mut [512]u8) callconv(.c) i32,
    write_fn ?*fn(u32, *[512]u8) callconv(.c) i32,
}

// Wired at boot by board.emmc2.init() — null before that point.
pub var sd_dev BlockDev = .{
    .read_fn = null,
    .write_fn = null,
}

const builtin = #import("builtin")

// See vfs.relocateOps / sys_call_table_relocate.
const LINEAR_MAP_BASE u64 = 0xFFFF000000000000

// Re-point sd_dev's function pointers to their high-mem (TTBR1)
// aliases. File syscalls run at EL1 with TTBR0 holding the *user*
// pgd; an indirect call through a low link-address pointer
// instruction-aborts because the user pgd does not map kernel low
// memory as executable. Mirrors vfs.relocateOps. `| BASE` is
// idempotent, so a double call is harmless. No-op on host builds
// (no TTBR split). The FAT32 backend's init() calls this before its
// first mount so every later read/write — kernel bring-up or
// syscall context — goes through the high alias.
pub fn relocate() void {
    if builtin.target.os.tag != .freestanding { return }
    if sd_dev.read_fn |f| {
        sd_dev.read_fn = #ptrFromInt(#intFromPtr(f) | LINEAR_MAP_BASE)
    }
    if sd_dev.write_fn |f| {
        sd_dev.write_fn = #ptrFromInt(#intFromPtr(f) | LINEAR_MAP_BASE)
    }
}