1 files changed, 38 insertions, 0 deletions
diff --git a/tinylfsr.c b/tinylfsr.c
new file mode 100644
index 0000000..fcea1a0
--- /dev/null
+++ b/tinylfsr.c
@@ -0,0 +1,38 @@
+#include <linux/kernel.h>
+
+enum poolinfo {
+	POOL_WORDS = 128,
+	POOL_WORDMASK = POOL_WORDS - 1,
+	POOL_BITS = POOL_WORDS * sizeof(u32) * 8,
+};
+
+static u32 input_pool_data[POOL_WORDS];
+
+static struct {
+	u16 add_ptr;
+} input_pool = { 0 };
+
+void mix_tinylfsr(const void *in, int nbytes)
+{
+	unsigned long i = input_pool.add_ptr, next;
+	const u8 *src = in;
+
+	/* Mix one byte at a time to simplify size handling and churn faster. */
+	while (nbytes--) {
+		/* The simplest possible max period LFSR.
+		 * This is morally equivalent to evaluating
+		 * the input as a polynomial at x^32.
+		 *
+		 * It can be evaluated in parallel, since i+0
+		 * only depends on i+0 and i+1. Thus i+0 depends
+		 * on i+0,i+1, i+1 depends on i+1 and i+2,
+		 * so we can absorb up to 126 bytes in parallel
+		 * if necessary.
+		 */
+		next = (i + 1) & POOL_WORDMASK;
+		input_pool_data[i] = *src++ ^ rol32(input_pool_data[i], 9) ^
+				(input_pool_data[next] << 15) ^ (input_pool_data[next] >> 24);
+		i = next;
+	}
+	input_pool.add_ptr = i;
+}