hammer: Add board_get_entropy function (and test)

This function will be used to generate some entropy using the
Clock Recovery System.

BRANCH=none
BUG=b:38486828
TEST=make BOARD=hammer -j tests
     ./util/flash_ec --board=hammer --image=build/hammer/test-entropy.bin
     EC console: runtest
TEST=Test fails when no USB connection is active
TEST=Test passes when USB connection is active
TEST=Pasting the values into:
     tr ';' '\n' | awk 'BEGIN { e = 0; tot=16384.0 }
                  { p = $1/tot; if (p > 0) { e -= p*log(p)/log(2) } }
                  END { print e }'
     shows an entropy > 4 bits per sample.

Change-Id: I2363c7bce42c72c33ef0bf3f099d709ee9c13d13
Reviewed-on: https://chromium-review.googlesource.com/518608
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/rollback.h b/include/rollback.h
index e33403b330..51f945e089 100644
--- a/include/rollback.h
+++ b/include/rollback.h
@@ -44,6 +44,19 @@ int rollback_add_entropy(uint8_t *data, unsigned int len);
  */
 int rollback_lock(void);
 
+/**
+ * Obtain some weak entropy (i.e. not guaranteed to be high quality), based on
+ * sensors or timing events.
+ *
+ * Must be defined if CONFIG_ROLLBACK_SECRET_SIZE is set. May sleep.
+ *
+ * @param buffer	Buffer to fill with entropy.
+ * @param len		Buffer length.
+ *
+ * @return true if the buffer was filled, false on error.
+ */
+int board_get_entropy(void *buffer, int len);
+
 #endif
 
 #endif  /* __CROS_EC_ROLLBACK_H */