SPI: send bootstub status in version request (#1492)

* mv first

* switch to crc8

* bootstub

* test

* cleanup

* little more

* misra

---------

Co-authored-by: Comma Device <device@comma.ai>

python/spi.py

@@ -12,6 +12,7 @@ from typing import List, Optional
 
 from .base import BaseHandle, BaseSTBootloaderHandle, TIMEOUT
 from .constants import McuType, MCU_TYPE_BY_IDCODE
+from .utils import crc8_pedal
 
 try:
   import spidev
@@ -175,25 +176,23 @@ class PandaSpiHandle(BaseHandle):
       logging.debug("- waiting for echo")
       start = time.monotonic()
       while True:
-        r = spi.readbytes(len(vers_str))
-        if bytes(r) == vers_str:
+        version_bytes = spi.readbytes(len(vers_str) + 2)
+        if bytes(version_bytes).startswith(vers_str):
           break
         if (time.monotonic() - start) > 0.5:
-          raise PandaSpiException("timed out waiting for version echo")
+          raise PandaSpiMissingAck
 
-      # get response
-      logging.debug("- receiving response")
-      b = bytes(spi.readbytes(2))
-      rlen = struct.unpack("<H", b)[0]
+      rlen = struct.unpack("<H", bytes(version_bytes[-2:]))[0]
       if rlen > 1000:
         raise PandaSpiException("response length greater than max")
 
-      dat = bytes(spi.readbytes(rlen))
-      resp = dat[:rlen//2]
-      resp_comp = dat[rlen//2:]
-      if resp != bytes([x ^ 0xff for x in resp_comp]):
-        raise PandaSpiException("data complement doesn't match")
-      return resp
+      # get response
+      dat = spi.readbytes(rlen + 1)
+      resp = dat[:-1]
+      calculated_crc = crc8_pedal(bytes(version_bytes + resp))
+      if calculated_crc != dat[-1]:
+        raise PandaSpiBadChecksum
+      return bytes(resp)
 
     exc = PandaSpiException()
     with self.dev.acquire() as spi:

python/utils.py

@@ -0,0 +1,12 @@
+def crc8_pedal(data):
+  crc = 0xFF    # standard init value
+  poly = 0xD5   # standard crc8: x8+x7+x6+x4+x2+1
+  size = len(data)
+  for i in range(size - 1, -1, -1):
+    crc ^= data[i]
+    for _ in range(8):
+      if ((crc & 0x80) != 0):
+        crc = ((crc << 1) ^ poly) & 0xFF
+      else:
+        crc <<= 1
+  return crc