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, 60 insertions, 0 deletions

diff --git a/test/entropy.c b/test/entropy.c
new file mode 100644
index 0000000000..f01a3dd0a0
--- /dev/null
+++ b/test/entropy.c
@@ -0,0 +1,60 @@
+/* Copyright 2017 The Chromium OS Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ *
+ * Tests entropy source.
+ */
+
+#include "console.h"
+#include "common.h"
+#include "rollback.h"
+#include "test_util.h"
+#include "timer.h"
+#include "util.h"
+#include "watchdog.h"
+
+static int buckets[256];
+
+void run_test(void)
+{
+	const int loopcount = 512;
+
+	uint8_t buffer[32];
+	timestamp_t t0, t1;
+	int i, j;
+
+	memset(buckets, 0, sizeof(buckets));
+
+	for (i = 0; i < loopcount; i++) {
+		t0 = get_time();
+		if (!board_get_entropy(buffer, sizeof(buffer))) {
+			ccprintf("Cannot get entropy\n");
+			test_fail();
+			return;
+		}
+		t1 = get_time();
+		if (i == 0)
+			ccprintf("Got %d bytes in %ld us\n",
+				sizeof(buffer), t1.val - t0.val);
+
+		for (j = 0; j < sizeof(buffer); j++)
+			buckets[buffer[j]]++;
+
+		watchdog_reload();
+	}
+
+	ccprintf("Total count: %d\n", loopcount * sizeof(buffer));
+	ccprintf("Buckets: ");
+	for (j = 0; j < 256; j++) {
+		ccprintf("%d;", buckets[j]);
+		cflush();
+	}
+	ccprintf("\n");
+	/*
+	 * From the data above, entropy can be obtained with this command:
+	 * tr ';' '\n' | awk 'BEGIN { e = 0; tot=16384.0 }
+		{ p = $1/tot; if (p > 0) { e -= p*log(p)/log(2) } }
+		END { print e }'
+	 */
+	test_pass();
+}