docs/system: riscv: Update Microchip Icicle Kit for direct kernel boot

This adds a new section in the documentation to demonstrate how to
use the new direct kernel boot feature for Microchip Icicle Kit,
other than the HSS bootflow, using an upstream U-Boot v2021.07 image
as an example.

It also updates the truth table to have a new '-dtb' column which is
required by direct kernel boot.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210706095045.1917913-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/riscv/microchip-icicle-kit.rst

@@ -47,13 +47,13 @@ The user provided DTB should have the following requirements:
 
 QEMU follows below truth table to select which payload to execute:
 
-=====  ========== =======
--bios     -kernel payload
-=====  ========== =======
-    N           N     HSS
-    Y  don't care     HSS
-    N           Y  kernel
-=====  ========== =======
+===== ========== ========== =======
+-bios    -kernel       -dtb payload
+===== ========== ========== =======
+    N          N don't care     HSS
+    Y don't care don't care     HSS
+    N          Y          Y  kernel
+===== ========== ========== =======
 
 The memory is set to 1537 MiB by default which is the minimum required high
 memory size by HSS. A sanity check on ram size is performed in the machine
@@ -106,4 +106,44 @@ HSS output is on the first serial port (stdio) and U-Boot outputs on the
 second serial port. U-Boot will automatically load the Linux kernel from
 the SD card image.
 
+Direct Kernel Boot
+------------------
+
+Sometimes we just want to test booting a new kernel, and transforming the
+kernel image to the format required by the HSS bootflow is tedious. We can
+use '-kernel' for direct kernel booting just like other RISC-V machines do.
+
+In this mode, the OpenSBI fw_dynamic BIOS image for 'generic' platform is
+used to boot an S-mode payload like U-Boot or OS kernel directly.
+
+For example, the following commands show building a U-Boot image from U-Boot
+mainline v2021.07 for the Microchip Icicle Kit board:
+
+.. code-block:: bash
+
+  $ export CROSS_COMPILE=riscv64-linux-
+  $ make microchip_mpfs_icicle_defconfig
+
+Then we can boot the machine by:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M microchip-icicle-kit -smp 5 -m 2G \
+      -sd path/to/sdcard.img \
+      -nic user,model=cadence_gem \
+      -nic tap,ifname=tap,model=cadence_gem,script=no \
+      -display none -serial stdio \
+      -kernel path/to/u-boot/build/dir/u-boot.bin \
+      -dtb path/to/u-boot/build/dir/u-boot.dtb
+
+CAVEATS:
+
+* Check the "stdout-path" property in the /chosen node in the DTB to determine
+  which serial port is used for the serial console, e.g.: if the console is set
+  to the second serial port, change to use "-serial null -serial stdio".
+* The default U-Boot configuration uses CONFIG_OF_SEPARATE hence the ELF image
+  ``u-boot`` cannot be passed to "-kernel" as it does not contain the DTB hence
+  ``u-boot.bin`` has to be used which does contain one. To use the ELF image,
+  we need to change to CONFIG_OF_EMBED or CONFIG_OF_PRIOR_STAGE.
+
 .. _HSS: https://github.com/polarfire-soc/hart-software-services