chestnut: fix flashing on old FW (#38639)

openpilot/system/hardware/chestnut/flash.py

@@ -33,6 +33,7 @@ USBDEVFS_SETCONFIGURATION = 0x80045505
 USBDEVFS_CLAIMINTERFACE = 0x8004550F
 USBDEVFS_RESET = 0x5514
 USBDEVFS_CLEAR_HALT = 0x80045515
+MAX_REGISTER_READ_SIZE = 255
 
 _deadline = float("inf")
 
@@ -146,6 +147,7 @@ def claim_interface(path, setup=False):
 class Flash:
   def __init__(self):
     self.fd = -1
+    self.max_register_read_size = MAX_REGISTER_READ_SIZE
 
   def close(self):
     if self.fd >= 0:
@@ -160,6 +162,9 @@ class Flash:
       if in_rom_bootloader(vid_pid, product):
         raise RomFallback("chestnut fell back to the ROM bootloader")
       if path is not None:
+        speed = int(open(path + "/speed").read())
+        # USB2 firmware truncates larger reads to one full packet without a terminating ZLP.
+        self.max_register_read_size = 64 if speed < 5000 else MAX_REGISTER_READ_SIZE
         self.fd = claim_interface(path)
         return
       time.sleep(0.1)
@@ -231,8 +236,8 @@ class Flash:
     while len(out) < length:
       n = min(4096, length - len(out))
       self.transaction(0x03, addr + len(out), max(4096, n))
-      for off in range(0, n, 255):
-        out += self.reg_read(0x7000 + off, min(255, n - off))
+      for off in range(0, n, self.max_register_read_size):
+        out += self.reg_read(0x7000 + off, min(self.max_register_read_size, n - off))
     return bytes(out)
 
   def erase_sector(self, addr):