Script to patch an Android recovery.img file, changing the qcom,qpnp-rtc-write Device Tree property (tested with an Android 5.1 image based on kernel 3.4 for a Nexus 5 (rev_11)).

patch-rtc.py

#!/usr/bin/env python
"""
Patch a recovery image to enable RTC writing

Unfortunately it has not the intended effect (tested from recovery):
~ # /system/xbin/hwclock -w
hwclock: RTC_SET_TIME: Operation not permitted

<3>[   25.821247] spmi_pmic_arb fc4cf000.qcom,spmi: pmic_arb_wait_for_done: transaction denied (0x5)
<3>[   25.821338] qcom,qpnp-rtc qpnp-rtc-ee162000: SPMI write failed
<3>[   25.821408] qcom,qpnp-rtc qpnp-rtc-ee162000: Write to RTC reg failed
"""
__author__ = "Peter Wu"
__email__ = "peter@lekensteyn.nl"
__license__ = "MIT"

import argparse, hashlib, logging, struct, sys
_logger = logging.getLogger(__name__)

def check(actual, expected, message):
    if expected != actual:
        raise RuntimeError("CHECK (%s == %s) failed: %s" % \
                (expected, actual, message))

# Expected page size
PAGE_SIZE = 2048

uint32_le_type = struct.Struct("<I") # For boot header
uint32_be_type = struct.Struct(">I") # For FDT
# unsigned 32-bit integers
KERNEL_SIZE_OFFSET = 8
RAMDISK_SIZE_OFFSET = 16
SECOND_SIZE_OFFSET = 24
PAGE_SIZE_OFFSET = 36

CHECKSUM_OFFSET = 576
CHECKSUM_LENGTH = 20

# Node name containing the qcom,qpnp-rtc-write property.
QCOM_RTC_NODE_NAME = b"qcom,pm8941_rtc"
# offset in structure for qcom,qpnp-rtc-write int property
QPNP_RTC_WRITE_PROP_NAMEOFF = 0x00001fa7

# https://github.com/we3des/psychic-octo-robot/wiki/Android-boot.img-zImage-format
# https://github.com/android/platform_system_core/blob/master/mkbootimg/bootimg.h
def parse_boot_img_header(block):
    check(block.startswith(b"ANDROID!"), True, "Boot magic")

    kernel_size, = uint32_le_type.unpack_from(block, KERNEL_SIZE_OFFSET)
    ramdisk_size, = uint32_le_type.unpack_from(block, RAMDISK_SIZE_OFFSET)
    second_size, = uint32_le_type.unpack_from(block, SECOND_SIZE_OFFSET)

    page_size, = uint32_le_type.unpack_from(block, PAGE_SIZE_OFFSET)
    check(page_size, PAGE_SIZE, "page size")

    checksum = block[CHECKSUM_OFFSET:CHECKSUM_OFFSET+CHECKSUM_LENGTH]

    return kernel_size, ramdisk_size, second_size, checksum

# SHA1(kernel | kernel_size | initrd | initrd_size | extra | extra_size)
def calculate_checksum(kernel, ramdisk, second):
    h = hashlib.sha1()
    for item in (kernel, ramdisk, second):
        h.update(item)
        h.update(uint32_le_type.pack(len(item)))
    return h.digest()

def calc_padding_size(size, block_size=4):
    """Returns the needed padding count to align data at word size."""
    return (block_size - (size % block_size)) % block_size

def print_size(label, size):
    _logger.info("%s", "{0:<19} {1:#010x} ({1})".format(label + ":", size))

def read_aligned(f, size):
    data = f.read(size)
    padding_size = calc_padding_size(size, PAGE_SIZE)
    return data, f.read(padding_size)

def read_image(image_filename):
    with open(image_filename, "rb") as f:
        header = f.read(PAGE_SIZE)
        kernel_size, ramdisk_size, second_size, checksum = \
                parse_boot_img_header(header)

        print_size("Kernel size", kernel_size)
        print_size("Ramdisk size", ramdisk_size)
        print_size("Second stage size", second_size)

        kernel, kernel_padding = read_aligned(f, kernel_size)
        ramdisk, ramdisk_padding = read_aligned(f, ramdisk_size)
        second, second_padding = read_aligned(f, second_size)
        checksum_expected = calculate_checksum(kernel, ramdisk, second)
        check(checksum, checksum_expected, "Invalid checksum")

        trailer = f.read()
        check(0, len(trailer), "expected no trailing data")

        return [header, kernel, kernel_padding, ramdisk, ramdisk_padding,
                second, second_padding]

def write_image(image_filename, blocks):
    with open(image_filename, "wb") as f:
        for block in blocks:
            f.write(block)


# Scan for structure block in FDT
# https://www.power.org/wp-content/uploads/2012/06/Power_ePAPR_APPROVED_v1.1.pdf#page=91
def find_structure_block(fdt, node_name, pos=0):
    assert pos % 4 == 0
    byteseq = b'\x00\x00\x00\x01'
    byteseq += node_name
    byteseq += b'\x00' * calc_padding_size(len(node_name))
    _logger.debug("Looking for %s in %d bytes", byteseq, len(fdt))

    # For each possible FDT_BEGIN_NODE, find properties.
    while True:
        pos = fdt.find(byteseq, pos)
        _logger.debug("FDT_BEGIN_NODE query result: %d", pos)
        if pos == -1:
            break
        pos += len(byteseq) # skip FDT_BEGIN_NODE

        while True:
            token_type, = uint32_be_type.unpack_from(fdt, pos)
            _logger.debug("FDT token type: %#010x", token_type)
            pos += 4 # skip token type

            if token_type == 2: # FDT_END_NODE
                break # Nothing interesting, look for next node.
            elif token_type == 4: # FDT_NOP
                pass # ignore, no data
            elif token_type == 3: # FDT_PROP
                # Scan for property (uint32_t len, namelen; uint8_t value[len])
                prop_len, = uint32_be_type.unpack_from(fdt, pos)
                prop_nameoff, = uint32_be_type.unpack_from(fdt, pos + 4)
                pos += 8
                _logger.debug("Prop len=%d, nameoff=%d", prop_len, prop_nameoff)
                if prop_nameoff == QPNP_RTC_WRITE_PROP_NAMEOFF:
                    return pos
                # skip value and padding
                pos += prop_len + calc_padding_size(prop_len)
            else:
                # Perhaps we did not run into a FDT node...
                _logger.warning("Unexpected token type %d", token_type)
                break
    return pos

def iter_props(kernel):
    pos = 0
    while pos != -1:
        pos = find_structure_block(kernel, QCOM_RTC_NODE_NAME, pos)
        if pos != -1:
            _logger.info("Found property value at offset %d", pos)
            yield pos

def patch_kernel(kernel, new_value):
    kernel = bytearray(kernel)
    for pos in iter_props(kernel):
        value, = uint32_be_type.unpack_from(kernel, pos)
        if value != new_value:
            _logger.info("Setting qcom,qpnp-rtc-write = <%#x>", 1)
            kernel[pos + 3] = new_value
        else:
            _logger.info("Not changing qcom,qpnp-rtc-write = <%#x>", value)
    return kernel

parser = argparse.ArgumentParser()
parser.add_argument("-d", "--debug", action="store_true",
        help="Enable verbose debug logging")
parser.add_argument("-n", "--dry-run", action="store_true",
        help="Avoid writing the file")
parser.add_argument("image_file", help="recovery.img file")
parser.add_argument("enable_write", type=int, choices=(0,1),
        help="Enable or disable RTC writing")

def main():
    args = parser.parse_args()
    logging.basicConfig(level=logging.DEBUG if args.debug else logging.INFO,
            format="%(name)s: %(message)s")
    image_filename  = args.image_file
    new_value = args.enable_write

    blocks = read_image(image_filename)
    header = blocks[0]
    kernel = blocks[1]
    ramdisk = blocks[3]
    second = blocks[5]

    kernel = patch_kernel(kernel, new_value)
    checksum = calculate_checksum(kernel, ramdisk, second)
    header = header[0:CHECKSUM_OFFSET] + checksum + \
            header[CHECKSUM_OFFSET+CHECKSUM_LENGTH:]

    blocks[0:2] = header, kernel
    if not args.dry_run:
        write_image(image_filename, blocks)
    else:
        _logger.info("Would write new image %s", image_filename)

if __name__ == '__main__':
    main()

This can patch boot image ?

Can you share your patched image or the kernel source for me ? My nexus reboot every time back to 1970... (sorry my bad english~)