doc: sifive: fu540: Update QEMU instruction when using U-Boot as the payload

Document that when U-Boot v2020.01 (or higher) is used as the payload,
we need adjust the instructions a little bit when testing OpenSBI with
QEMU 'sifive_u' machine.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

diff --git a/docs/platform/sifive_fu540.md b/docs/platform/sifive_fu540.md
index 0a9f21e..94eb81f 100644 (file)
--- a/docs/platform/sifive_fu540.md
+++ b/docs/platform/sifive_fu540.md
@@ -183,3 +183,13 @@ same instructions above, with the exception of not passing FW_PAYLOAD_FDT_PATH.
 This is because QEMU generates a device tree blob on the fly based on the
 command line parameters and it's compatible with the one used in the upstream
 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:
+
+```
+make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<u-boot_build_dir>/u-boot.bin
+```