src/firmware/gigaboot/cpp/gen_gbl_dt_cc.py

# Copyright 2024 The Fuchsia Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""Generates device tree blob for booting GBL"""

# GBL currently relies on device tree for passing platform specific data such
# as ZBI items, permanent attributes etc. The script generates a software blob
# containing a pre-generated device tree with placeholder bytes for poplulating
# these data. Once the custom GBL EFI protocols are implemented which support
# these, the script and the device tree mechanism can be removed.
#
# The scripts can't be run by Fuchsia build system because it requires the
# pylibfdt module which isn't included.

import os
import pathlib
import subprocess
import textwrap

from libfdt import FdtSw

SCRIPT_DIR = pathlib.Path(os.path.dirname(os.path.realpath(__file__)))

# Value from `sizeof(AvbAtxPermanentAttributes)`.
PERM_ATTR_SIZE = 1052

# SHA256
PERM_ATTR_HASH_SIZE = 32

ZBI_BLOB_CAPACITY = 4096

if __name__ == "__main__":
    sw = FdtSw()
    sw.finish_reservemap()
    with sw.add_node(""):
        with sw.add_node("zircon"):
            sw.property("zbi-blob", b"\x00" * ZBI_BLOB_CAPACITY)
        with sw.add_node("gbl"):
            sw.property(
                "avb-cert-permanent-attributes", b"\x00" * PERM_ATTR_SIZE
            )
            sw.property(
                "avb-cert-permanent-attributes-hash",
                b"\x00" * PERM_ATTR_HASH_SIZE,
            )
            sw.property("stop-in-fastboot", b"\x00")
    fdt = sw.as_fdt()
    # Get the property offset relative to dt struct section
    zbi_blob = fdt.first_property_offset(fdt.subnode_offset(0, "zircon"))
    per_attr = fdt.first_property_offset(fdt.subnode_offset(0, "gbl"))
    per_attr_hash = fdt.next_property_offset(per_attr)
    stop_in_fastboot = fdt.next_property_offset(per_attr_hash)

    # Computes the absolute offset for property payloads relative to the start
    # of FDT. This is done by adding the DT struct offset and skipping the
    # `tag`, `len`, `nameoff` fields (12 bytes in total) in the property
    # structure:
    #
    # struct fdt_property {
    #   fdt32_t tag;
    #   fdt32_t len;
    #   fdt32_t nameoff;
    #   char data[0];
    # };
    zbi_blob += fdt.off_dt_struct() + 12
    per_attr += fdt.off_dt_struct() + 12
    per_attr_hash += fdt.off_dt_struct() + 12
    stop_in_fastboot += fdt.off_dt_struct() + 12

    cc_file = SCRIPT_DIR / "gbl_dt.cc"
    cc_file.write_text(
        textwrap.dedent(
            f"""// Copyright 2024 The Fuchsia Authors. All rights reserved.
            // Use of this source code is governed by a BSD-style license that can be
            // found in the LICENSE file.

            #include <span>

            // This file is generated by {os.path.basename(__file__)}
            namespace {{
            uint8_t dt[] = {{{''.join([f'0x{b:02x},' for b in fdt.as_bytearray()])}}};

            const size_t kZbiBlobOffset = {zbi_blob};
            const size_t kZbiBlobSize = {ZBI_BLOB_CAPACITY};
            const size_t kPermAttrOffset = {per_attr};
            const size_t kPermAttrSize = {PERM_ATTR_SIZE};
            const size_t kPermAttrHashOffset = {per_attr_hash};
            const size_t kPermAttrHashSize = {PERM_ATTR_HASH_SIZE};
            const size_t kStopInFastbootOffset = {stop_in_fastboot};
            }}

            namespace gigaboot {{
            std::span<uint8_t> GblFdt() {{
              return {{dt , sizeof(dt)}};
            }}

            std::span<uint8_t> GblFdtZbiBlob() {{
              return {{dt + kZbiBlobOffset, kZbiBlobSize}};
            }}

            std::span<uint8_t> GblFdtPermAttr() {{
              return {{dt + kPermAttrOffset, kPermAttrSize}};
            }}

            std::span<uint8_t> GblFdtPermAttrHash() {{
              return {{dt + kPermAttrHashOffset, kPermAttrHashSize}};
            }}

            std::span<uint8_t> GblFdtStopInFastboot() {{
              return {{dt + kStopInFastbootOffset, 1}};
            }}
            }}
            """
        )
    )

    subprocess.run(["fx", "format-code", f"--files={cc_file}"], cwd=SCRIPT_DIR)