spi_check: +TPL plaintext verification and optional --blob byte-match

Discovered the rkspi wrapper format during offline RC4 probing:
the RKNS-wrapper sector at 0x8000 is plaintext. Zero padding
fills 0x8200..0x85FF. Encoded metadata sits at 0x8600. The TPL
(DDR blob) starts at 0x8800 in plaintext -- not RC4 encrypted
as I first guessed.

New checks:
  - TPL entry signature (0x01 0x00 0x00 0x14 = b +4 skipping header)
    at offset 0x8800 -- catches silent TPL corruption
  - optional --blob <path>: byte-by-byte compare SPI[0x8800:+len(blob)]
    against a reference DDR blob file, reports sha256 + first-diff
    offset on mismatch

Validated against stock SPI with stock blob (PASS, sha 13c04c4f),
patched SPI with patched blob (PASS, sha 85799151), and the
cross-pair (FAIL with diagnostics).

Closes the remaining gap in phase-1 static validation -- now
we catch not just |image has no idbloader| but also |image has
the wrong DDR bytes|.

spi_check.py

@@ -19,18 +19,20 @@ Exit codes:
 
 Usage: rk3588_spi_check.py <spi-image.bin>
 """
-import struct
+import argparse, hashlib, struct
 import sys
 
 IDBL_OFFSET = 0x8000
 RKNS_MAGIC  = 0x534E4B52   # "RKNS" LE — rkspi boot header
+TPL_OFFSET  = 0x8800       # plaintext DDR blob (TPL) starts here in rkspi layout
+TPL_ENTRY   = bytes.fromhex("01000014")  # first 4 bytes of the RK3588 DDR blob entry
 PAYLOAD_END = 0x60000      # stock u-boot places next section here
 
 def die(code, msg):
     print(f"FAIL: {msg}", file=sys.stderr)
     sys.exit(code)
 
-def main(path):
+def main(path, blob_path=None):
     with open(path, "rb") as f:
         data = f.read()
     print(f"SPI image: {path}  size=0x{len(data):x} ({len(data)} B)")
@@ -63,11 +65,33 @@ def main(path):
     flag_like = struct.unpack_from("<I", w, 12)[0]
     print(f"    wrapper[+8]=0x{size_like:08x}  wrapper[+12]=0x{flag_like:08x}")
 
+    # TPL content check — the DDR blob lives in plaintext at 0x8800
+    tpl_prefix = data[TPL_OFFSET:TPL_OFFSET + 4]
+    if tpl_prefix != TPL_ENTRY:
+        die(4, f"TPL at 0x{TPL_OFFSET:x} does not start with expected blob entry "
+               f"{TPL_ENTRY.hex()}: got {tpl_prefix.hex()}")
+    print(f"OK  TPL blob entry signature at 0x{TPL_OFFSET:x}")
+
+    if blob_path:
+        with open(blob_path, "rb") as f:
+            blob = f.read()
+        tpl_region = data[TPL_OFFSET:TPL_OFFSET + len(blob)]
+        if tpl_region != blob:
+            # Find the first differing byte for diagnostics
+            diffs = [i for i in range(len(blob)) if i < len(tpl_region) and blob[i] != tpl_region[i]]
+            first_diff = diffs[0] if diffs else len(tpl_region)
+            die(5, f"TPL in SPI does NOT match blob file.\n"
+                   f"    blob sha256: {hashlib.sha256(blob).hexdigest()}\n"
+                   f"    SPI[0x{TPL_OFFSET:x}:+{len(blob)}] sha256: {hashlib.sha256(tpl_region).hexdigest()}\n"
+                   f"    first differing byte at blob offset 0x{first_diff:x}")
+        print(f"OK  TPL bytes match {blob_path}  sha256={hashlib.sha256(blob).hexdigest()[:16]}…  ({len(blob)} B)")
+
     print("\nPASS: image looks structurally sound. Safe to flash.")
     sys.exit(0)
 
 if __name__ == "__main__":
-    if len(sys.argv) != 2:
+    ap = argparse.ArgumentParser(description=__doc__)
-        print(__doc__)
+    ap.add_argument("spi", help="u-boot-rockchip-spi-*.bin to validate")
-        sys.exit(1)
+    ap.add_argument("--blob", help="optional raw DDR blob to byte-compare against the TPL region")
-    main(sys.argv[1])
+    a = ap.parse_args()
+    main(a.spi, a.blob)