lib/crc8: Add crc start value

To make the usage of this function more flexible, lets add the CRC start
value as parameter to this function. This way it can be used by other
functions requiring different start values than 0 as well.

For non-zero CRC start values to work, I've reworked the function a bit.
The new implementation is copied from the Linux version in
drivers/i2c/i2c-core.c / i2c_smbus_pec(). Which supports non-zero
CRC stating values.

I've double-checked that the results for zero starting values are
identical to the results from the original version of this function.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/tpm/tpm_tis_sandbox.c

@@ -217,7 +217,7 @@ static int sandbox_tpm_xfer(struct udevice *dev, const uint8_t *sendbuf,
 			rsk.struct_version = 2;
 			rsk.uid = ROLLBACK_SPACE_KERNEL_UID;
 			rsk.kernel_versions = 0;
-			rsk.crc8 = crc8((unsigned char *)&rsk,
+			rsk.crc8 = crc8(0, (unsigned char *)&rsk,
 					offsetof(struct rollback_space_kernel,
 						 crc8));
 			memcpy(data, &rsk, sizeof(rsk));

include/linux/crc8.h

@@ -14,10 +14,11 @@
  * This uses an x^8 + x^2 + x + 1 polynomial.  A table-based algorithm would
  * be faster, but for only a few bytes it isn't worth the code size
  *
+ * @crc_start: CRC8 start value
  * @vptr: Buffer to checksum
  * @len: Length of buffer in bytes
  * @return CRC8 checksum
  */
-unsigned int crc8(const unsigned char *vptr, int len);
+unsigned int crc8(unsigned int crc_start, const unsigned char *vptr, int len);
 
 #endif

lib/crc8.c

@@ -6,20 +6,27 @@
 
 #include "linux/crc8.h"
 
-unsigned int crc8(const unsigned char *vptr, int len)
+#define POLY	(0x1070U << 3)
+
+static unsigned char _crc8(unsigned short data)
 {
-	const unsigned char *data = vptr;
-	unsigned int crc = 0;
-	int i, j;
-
-	for (j = len; j; j--, data++) {
-		crc ^= (*data << 8);
-		for (i = 8; i; i--) {
-			if (crc & 0x8000)
-				crc ^= (0x1070 << 3);
-			crc <<= 1;
-		}
+	int i;
+
+	for (i = 0; i < 8; i++) {
+		if (data & 0x8000)
+			data = data ^ POLY;
+		data = data << 1;
 	}
 
-	return (crc >> 8) & 0xff;
+	return (unsigned char)(data >> 8);
+}
+
+unsigned int crc8(unsigned int crc, const unsigned char *vptr, int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++)
+		crc = _crc8((crc ^ vptr[i]) << 8);
+
+	return crc;
 }