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.nortm...@web.de>
---

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

 tools/mksunxiboot.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/tools/mksunxiboot.c b/tools/mksunxiboot.c
index 676d392..cb0865c 100644
--- a/tools/mksunxiboot.c
+++ b/tools/mksunxiboot.c
@@ -27,12 +27,29 @@ 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 */
 };
 
 #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 +150,10 @@ 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

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