[PATCH 3/3] docs/platform: sifive_fu540: Update U-Boot instructions

[Bin Meng]

From: Bin Meng <bin.meng at windriver.com>

U-Boot v2020.07 release adds SPL support to SiFive HiFive Unleashed.
Update the doc to mention that detailed build instructions are in the
U-Boot doc.

This also adds detailed command line description to show how to boot
U-Boot v2020.07 S-mode payload in the QEMU chapter.

Signed-off-by: Bin Meng <bin.meng at windriver.com>
---

 docs/platform/sifive_fu540.md | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/docs/platform/sifive_fu540.md b/docs/platform/sifive_fu540.md
index 9342846..31bdf09 100644
--- a/docs/platform/sifive_fu540.md
+++ b/docs/platform/sifive_fu540.md
@@ -41,13 +41,14 @@ make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<linux_build_directory>/arch/riscv/bo
 **U-Boot Payload**
 
 The command-line example here assumes that U-Boot was compiled using the
-sifive_fu540_defconfig configuration and with U-Boot v2020.01 (or higher).
-
-The detailed U-Boot booting guide is avaialble at [U-Boot].
+sifive_fu540_defconfig configuration and with U-Boot v2020.01, and up to
+v2020.07-rc3.
 
 ```
 make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<u-boot_build_dir>/u-boot-dtb.bin
 ```
+For U-Boot v2020.07-rc4 or later releases, SPL support was added in U-Boot.
+Please refer to the detailed U-Boot booting guide avaialble at [U-Boot].
 
 Flashing the OpenSBI firmware binary to storage media:
 ------------------------------------------------------
@@ -155,13 +156,37 @@ Linux kernel.
 When U-Boot v2020.01 (or higher) is used as the payload, as the SiFive FU540
 DTB for the real hardware is embedded in U-Boot binary itself, due to the same
 reason above, we need to switch the U-Boot sifive_fu540_defconfig configuration
-from CONFIG_OF_SEPARATE to CONFIG_OF_PRIOR_STAGE so that U-Boot uses the DTB
-generated by QEMU, and u-boot.bin should be used as the payload image, like:
+from **CONFIG_OF_SEPARATE** to **CONFIG_OF_PRIOR_STAGE** so that U-Boot uses the
+DTB generated by QEMU, and u-boot.bin should be used as the payload image, like:
 
 ```
 make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<u-boot_build_dir>/u-boot.bin
 ```
 
+U-Boot v2020.07 release added SPL support to SiFive HiFive Unleashed board,
+hence a build error will be seen after you switch to **CONFIG_OF_PRIOR_STAGE**.
+
+```
+./tools/mkimage: Can't open arch/riscv/dts/hifive-unleashed-a00.dtb: No such file or directory
+./tools/mkimage: failed to build FIT
+Makefile:1402: recipe for target 'u-boot.img' failed
+make: *** [u-boot.img] Error 1
+```
+
+The above errors can be safely ignored as we don't run U-Boot SPL under QEMU.
+
+Run:
+```
+qemu-system-riscv64 -M sifive_u -m 256M -nographic \
+	-bios build/platform/sifive/fu540/firmware/fw_payload.bin
+```
+or
+```
+qemu-system-riscv64 -M sifive_u -m 256M -nographic \
+	-bios build/platform/sifive/fu540/firmware/fw_jump.bin \
+	-kernel <uboot_build_dir>/u-boot.bin
+```
+
 While the real hardware operates at the 64-bit mode, it's possible for QEMU to
 test the 32-bit OpenSBI firmware. This can be helpful for testing 32-bit SiFive
 specific drivers.
-- 
2.7.4