[U-Boot] [PATCH v3 2/4] 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>
---

Changes in v3:
- adapted to use asm/arch/spl.h

Changes in v2:
- Rename field to fel_script_address, discard fel_data_size
- Clearing header fields is no longer needed, as mksunxiboot.c zeroes entire image first

 arch/arm/include/asm/arch-sunxi/spl.h | 19 ++++++++++++++++++-
 tools/mksunxiboot.c                   |  4 ++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/arch-sunxi/spl.h b/arch/arm/include/asm/arch-sunxi/spl.h
index 751de75..08fe32d 100644
--- a/arch/arm/include/asm/arch-sunxi/spl.h
+++ b/arch/arm/include/asm/arch-sunxi/spl.h
@@ -10,6 +10,8 @@
 #define	_ASM_ARCH_SPL_H_
 
 #define BOOT0_MAGIC		"eGON.BT0"
+#define SPL_SIGNATURE		"SPL" /* marks "sunxi" SPL header */
+#define SPL_HEADER_VERSION	1
 
 /* boot head definition from sun4i boot code */
 struct boot_file_head {
@@ -22,8 +24,23 @@ 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.)
 	 */
-	uint8_t pad[12];		/* align to 32 bytes */
+	union {
+		uint32_t pub_head_size;
+		uint8_t spl_signature[4];
+	};
+	uint32_t fel_script_address;
+	uint32_t reserved;		/* padding, align to 32 bytes */
 };
 
 #endif
diff --git a/tools/mksunxiboot.c b/tools/mksunxiboot.c
index 54f4d05..9c1c5b7 100644
--- a/tools/mksunxiboot.c
+++ b/tools/mksunxiboot.c
@@ -118,6 +118,10 @@ 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;
+
 	gen_check_sum(&img.header);
 
 	count = write(fd_out, &img, le32_to_cpu(img.header.length));
-- 
2.4.6