rockchip: mkimage: simplify start/size calculation for rc4_encode

The RC4 encoding works on full blocks, but the calculation of the
starting offset and size are needlessly complicated by using a
reference value known to be offset into a block by the size of the
header and then correcting for the (hard-coded) size of the header
(i.e. 4 bytes).

We change this over to use the RK_SPL_HDR_START directly (which is
known to be on a block boundary).

X-AffectedPlatforms: RK3399-Q7
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>

tools/rksd.c

@@ -43,8 +43,8 @@ static void rksd_set_header(void *buf,  struct stat *sbuf,  int ifd,
 	       RK_SPL_HDR_SIZE);
 
 	if (rkcommon_need_rc4_spl(params))
-		rkcommon_rc4_encode_spl(buf, RK_SPL_START - 4,
-					params->file_size - RK_SPL_START + 4);
+		rkcommon_rc4_encode_spl(buf, RK_SPL_HDR_START,
+					params->file_size - RK_SPL_HDR_START);
 }
 
 static int rksd_extract_subimage(void *buf,  struct image_tool_params *params)

tools/rkspi.c

@@ -49,8 +49,8 @@ static void rkspi_set_header(void *buf, struct stat *sbuf, int ifd,
 	       RK_SPL_HDR_SIZE);
 
 	if (rkcommon_need_rc4_spl(params))
-		rkcommon_rc4_encode_spl(buf, RK_SPL_START - 4,
-					params->file_size - RK_SPL_START + 4);
+		rkcommon_rc4_encode_spl(buf, RK_SPL_HDR_START,
+					params->file_size - RK_SPL_HDR_START);
 
 	/*
 	 * Spread the image out so we only use the first 2KB of each 4KB