docs: platform: Update descriptions for qemu/sifive_u support

With QEMU v4.2 release that has improved the emulation fidelity
of 'sifive_u' machine, OpenSBI v0.4 / U-Boot v2019.10-rc1 / Linux
kernel v5.3-rc2 images built for the SiFive HiFive Unleashed board
can be used out of the box without any special hack.

Update our documents to mention 'qemu/sifive_u' platform should
only be used with QEMU v4.1 or before, and it will be dropped
sometime in the future release.

Going forward, 'sifive/fu540' platform can be used on both real
hardware and QEMU v4.2+ 'sifive_u' machine.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

2 files changed, 20 insertions, 0 deletions

diff --git a/docs/platform/qemu_sifive_u.md b/docs/platform/qemu_sifive_u.md
index 48b30e4..b5c4f0e 100644
--- a/docs/platform/qemu_sifive_u.md
+++ b/docs/platform/qemu_sifive_u.md
@@ -7,6 +7,14 @@ platform.
 To build this platform specific library and firmwares, provide the
 *PLATFORM=qemu/sifive_u* parameter to the top level `make` command line.
 
+Note with QEMU v4.2 release, the QEMU *sifive_u* machine has been updated to
+closely match the SiFive HiFive Unleashed hardware and can therefore run the
+same firmware as what gets loaded onto the board, and OpenSBI's *qemu/sifive_u*
+platform should only be used with QEMU v4.1 release or before.
+
+The special *qemu/sifive_u* platform support will be dropped in the future
+OpenSBI release.
+
 Platform Options
 ----------------
 
diff --git a/docs/platform/sifive_fu540.md b/docs/platform/sifive_fu540.md
index 6df904d..9fb45d6 100644
--- a/docs/platform/sifive_fu540.md
+++ b/docs/platform/sifive_fu540.md
@@ -4,6 +4,9 @@ The FU540-C000 is the world’s first 4+1 64-bit RISC-V SoC from SiFive.
 The HiFive Unleashed development platform is based on FU540-C000 and capable
 of running Linux.
 
+With QEMU v4.2 or above release, the 'sifive_u' machine can be used to test
+OpenSBI image built for the real hardware as well.
+
 To build platform specific library and firmwares, provide the
 *PLATFORM=sifive/fu540* parameter to the top level `make` command.
 
@@ -201,3 +204,12 @@ At U-Boot prompt execute the following boot command to boot Linux.
 ```
 bootm ${kernel_addr_r} - ${fdt_addr_r}
 ```
+
+QEMU Specific Instructions
+--------------------------
+If you want to test OpenSBI with QEMU 'sifive_u' machine, please follow the
+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.