[U-Boot] [PATCH 3/3] rockchip: rk3188: add README.rockchip paragraph describing sd boot

[Heiko Stuebner]

Building sd images for rk3188 requires more steps due to the needed split
into TPL and SPL as loaders. Describe how to build an image for it in a
separate paragraph in the READER.rockchip file.

Signed-off-by: Heiko Stuebner <heiko at sntech.de>
---
 doc/README.rockchip | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/doc/README.rockchip b/doc/README.rockchip
index 186a1a007e..cb81efd4bf 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -147,6 +147,32 @@ For evb_rk3036 board:
 Note: rk3036 SDMMC and debug uart use the same iomux, so if you boot from SD, the
       debug uart must be disabled
 
+
+Booting from an SD card on RK3188
+=================================
+
+For rk3188 boards the general storage onto the card stays the same as
+described above, but the image creation needs a bit more care.
+
+The bootrom of rk3188 expects to find a small 1kb loader which returns
+control to the bootrom, after which it will load the real loader, which
+can then be up to 29kb in size and does the regular ddr init.
+
+Additionally the rk3188 requires everything the bootrom loads to be
+rc4-encrypted. Except for the very first stage the bootrom always reads
+and decodes 2kb pages, so files should be sized accordingly.
+
+# copy tpl, pad to 1020 bytes and append spl
+cat tpl/u-boot-tpl.bin > tplspl.bin
+truncate -s 1020 tplspl.bin
+cat spl/u-boot-spl.bin >> tplspl.bin
+tools/mkimage -n rk3188 -T rksd -d tplspl.bin out
+
+# truncate, encode and append u-boot.bin
+truncate -s %2048 u-boot.bin
+cat u-boot.bin | split -b 512 --filter='openssl rc4 -K 7C4E0304550509072D2C7B38170D1711' >> out
+
+
 Using fastboot on rk3288
 ========================
 - Write GPT partition layout to mmc device which fastboot want to use it to
-- 
2.11.0