1 files changed, 63 insertions, 0 deletions
diff --git a/src/appdesc.zig b/src/appdesc.zig
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index 0000000..bec2915
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+++ b/src/appdesc.zig
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+//! esp_app_desc_t, the 256-byte block the second-stage bootloader reads from image offset 0x20.
+//!
+//! Two facts about it were established by flashing deliberately broken images at this board:
+//!   * the magic word is never validated in a default build - the descriptor is trusted purely by
+//!     position, so an image with ordinary rodata there can boot;
+//!   * what the loader actually reads is min/max_efuse_blk_rev_full at offsets 0xB0/0xB2, and
+//!     `IS_FIELD_SET()` treats zero as "unset" (bootloader_common_loader.c:102-112).
+//!
+//! Everything past 0xB5 is reserved padding, so `minimal` stops there and saves 72 bytes of flash.
+//! `full` keeps all 256 so that `esptool image-info` can parse it.
+
+const config = @import("config");
+
+pub const magic: u32 = 0xABCD5432;
+
+pub const Minimal = extern struct {
+    magic_word: u32 = magic,
+    secure_version: u32 = 0,
+    reserv1: [2]u32 = .{ 0, 0 },
+    version: [32]u8,
+    project_name: [32]u8,
+    time: [16]u8 = @splat(0),
+    date: [16]u8 = @splat(0),
+    idf_ver: [32]u8 = @splat(0),
+    app_elf_sha256: [32]u8 = @splat(0),
+    min_efuse_blk_rev_full: u16,
+    max_efuse_blk_rev_full: u16,
+    mmu_page_size: u8 = 16, // log2(64 KiB); fixed on the P4, which has no configurable MMU page
+    reserv3: [3]u8 = @splat(0),
+};
+
+pub const Full = extern struct {
+    head: Minimal,
+    reserv2: [18]u32 = @splat(0),
+};
+
+fn str(comptime n: usize, comptime s: []const u8) [n]u8 {
+    var out: [n]u8 = @splat(0);
+    @memcpy(out[0..s.len], s);
+    return out;
+}
+
+const head: Minimal = .{
+    .version = str(32, "0.1"),
+    .project_name = str(32, "zig-p4"),
+    // The eFuse *block* revision, which has nothing to do with the silicon revision window that
+    // -Dmin-rev sets in the image header. Deriving one from the other made `-Dmin-rev=150` emit an
+    // image the bootloader refuses with "Image requires efuse blk rev >= v0.50". Zero means unset,
+    // which is what `IS_FIELD_SET()` checks for (bootloader_common_loader.c:102-112).
+    .min_efuse_blk_rev_full = 0,
+    .max_efuse_blk_rev_full = 0,
+};
+
+/// Placed first in .flash.rodata by the linker script, so it lands at image offset 0x20.
+///
+/// `export` is load-bearing: a `comptime _ = descriptor;` reference is enough in Debug but not
+/// under ReleaseSmall, where the constant is folded away, the section disappears, `KEEP` has
+/// nothing to keep, and the image ends up with one mapped segment starting at the wrong offset.
+/// An exported symbol is always emitted.
+pub export const esp_app_desc linksection(".rodata.appdesc") = if (config.full_descriptor)
+    Full{ .head = head }
+else
+    head;