Add OTP support

One Time Programmable memory can be used to store permanent data like
serial numbers.
Reorganize the code to support writing serial number to OTP, in
addition to pstate (if using its own memory bank) or autogenerate from
unique id (hammer).

+ Add CONFIG_OTP to enable OTP code
+ Add CONFIG_SERIALNO_LEN to indicate the size of the serial number
string.  Currently set to 28, when USB serial number is needed.
+ Expose flash_read|write_pstate_serial and add otp_read|write_serail,
remove more generic flash_read|write_serial.
+ Make board_read|write_serial generic, declared outside of USB subsystem.

Priority order to read|write serial string:
- board definition (like hammer)
- pstate location, if stored in its private memory bank
- otp area
If none of these methods are available, a compilation error is raised.

BUG=chromium:746471
BRANCH=none
TEST=compile

Change-Id: I3d16125a6c0f424fb30e38123e63cf074b3cb2d3
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/580289
Reviewed-by: Nick Sanders <nsanders@chromium.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/include/system.h b/include/system.h
index a4595b3651..e432a994c3 100644
--- a/include/system.h
+++ b/include/system.h
@@ -287,10 +287,15 @@ int system_get_chip_unique_id(uint8_t **id);
 
 /**
  * Optional board-level callback functions to read a unique serial number per
- * chip. Default implementation reads from flash (flash_read_serial).
+ * chip. Default implementation reads from flash/otp (flash/otp_read_serial).
  */
 const char *board_read_serial(void) __attribute__((weak));
 
+/**
+ * Optional board-level callback functions to write a unique serial number per
+ * chip. Default implementation reads from flash/otp (flash/otp_write_serial).
+ */
+int board_write_serial(const char *serial) __attribute__((weak));
 /*
  * Common bbram entries. Chips don't necessarily need to implement
  * all of these, error will be returned from system_get/set_bbram if