[U-Boot] [PATCH] sunxi: (mksunxiboot) signature to indicate "sunxi" SPL variant

[Bernhard Nortmann]

This patch follows up on a discussion of ways to improve support
for the sunxi FEL ("USB boot") mechanism, especially with regard
to boot scripts, see:
https://groups.google.com/d/msg/linux-sunxi/wBEGUoLNRro/rHGq6nSYCQAJ

The idea is to convert the (currently unused) "pad" bytes in the
SPL header into an area where data can be passed to U-Boot. To
do this safely, we have to make sure that we're actually using
our "sunxi" flavor of the SPL, and not the Allwinner boot0.

The modified mksunxiboot introduces a special signature to the
SPL header in place of the "pub_head_size" field. This can be
used to reliably distinguish between compatible versions of sunxi
SPL and anything else (older variants or Allwinner's boot0).

Signed-off-by: Bernhard Nortmann <bernhard.nortmann at web.de>
---

 tools/mksunxiboot.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/tools/mksunxiboot.c b/tools/mksunxiboot.c
index 3361251..58bc5c7 100644
--- a/tools/mksunxiboot.c
+++ b/tools/mksunxiboot.c
@@ -27,12 +27,31 @@ struct boot_file_head {
 	 * by the boot ROM. To be compatible with Allwinner tools we
 	 * would need to implement the proper fields here instead of
 	 * padding.
+	 *
+	 * Actually we want the ability to recognize our "sunxi" variant
+	 * of the SPL. To do so, let's place a special signature into the
+	 * "pub_head_size" field. We can reasonably expect Allwinner's
+	 * boot0 to always have the upper 16 bits of this set to 0 (after
+	 * all the value shouldn't be larger than the limit imposed by
+	 * SRAM size).
+	 * If the signature is present (at 0x14), then we know it's safe
+	 * to use the remaining 8 bytes (at 0x18) for our own purposes.
+	 * (E.g. sunxi-tools "fel" utility can pass information there.)
+	 *
+	 * The overall header size still sums up to 32 bytes.
 	 */
-	uint8_t pad[12];		/* align to 32 bytes */
+	union {
+		uint32_t pub_head_size;
+		uint8_t spl_signature[4];
+	};
+	uint32_t fel_data_address;
+	uint32_t fel_data_size;
 };
 
 #define BOOT0_MAGIC                     "eGON.BT0"
 #define STAMP_VALUE                     0x5F0A6C39
+#define SPL_SIGNATURE			"SPL" /* marks "sunxi" header */
+#define SPL_HEADER_VERSION		1
 
 /* check sum functon from sun4i boot code */
 int gen_check_sum(struct boot_file_head *head_p)
@@ -133,6 +152,12 @@ int main(int argc, char *argv[])
 		ALIGN(file_size + sizeof(struct boot_file_head), BLOCK_SIZE);
 	img.header.b_instruction = cpu_to_le32(img.header.b_instruction);
 	img.header.length = cpu_to_le32(img.header.length);
+
+	memcpy(img.header.spl_signature, SPL_SIGNATURE, 3); /* "sunxi" marker */
+	img.header.spl_signature[3] = SPL_HEADER_VERSION;
+	img.header.fel_data_address = 0; /* ensure fields are zeroed */
+	img.header.fel_data_size = 0;
+
 	gen_check_sum(&img.header);
 
 	count = write(fd_out, &img, le32_to_cpu(img.header.length));
-- 
2.4.6