docs: platform: Add documentation for Spike platform

This patch adds documentation to build and run OpenSBI on
Spike simulator and QEMU Spike machine.

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

docs/platform/platform.md

@@ -23,6 +23,8 @@ OpenSBI currently supports the following virtual and hardware platforms:
 
 * **T-HEAD C910**: Platform support for the T-HEAD C910 Processor.
 
+* **Spike**: Platform support for the Spike emulator.
+
 The code for these supported platforms can be used as example to implement
 support for other platforms. The *platform/template* directory also provides
 template files for implementing support for a new platform. The *object.mk*,
@@ -34,3 +36,4 @@ facilitate the implementation.
 [ariane-fpga.md]: ariane-fpga.md
 [andes_ae350.md]: andes-ae350.md
 [thead-c910.md]: thead-c910.md
+[spike.md]: spike.md

docs/platform/spike.md

@@ -0,0 +1,89 @@
+Spike Simulator Platform
+========================
+
+The **Spike** is a RISC-V ISA simulator which implements a functional model
+of one or more RISC-V harts. The **Spike** compatible virtual platform is
+also available on QEMU. In fact, we can use same OpenSBI firmware binaries
+on **Spike** simulator and QEMU Spike machine.
+
+For more details, refer [Spike on GitHub](https://github.com/riscv/riscv-isa-sim)
+
+To build the platform-specific library and firmware images, provide the
+*PLATFORM=spike* parameter to the top level `make` command.
+
+Platform Options
+----------------
+
+The *Spike* platform does not have any platform-specific options.
+
+Execution on Spike Simulator
+----------------------------
+
+**No Payload Case**
+
+Build:
+```
+make PLATFORM=spike
+```
+
+Run:
+```
+spike build/platform/spike/firmware/fw_payload.elf
+```
+
+**Linux Kernel Payload**
+
+Note: We assume that the Linux kernel is compiled using
+*arch/riscv/configs/defconfig*.
+
+Build:
+```
+make PLATFORM=spike FW_PAYLOAD_PATH=<linux_build_directory>/arch/riscv/boot/Image
+```
+
+Run:
+```
+spike --initrd <path_to_cpio_ramdisk> build/platform/spike/firmware/fw_payload.elf
+```
+
+Execution on QEMU RISC-V 64-bit
+-------------------------------
+
+**No Payload Case**
+
+Build:
+```
+make PLATFORM=spike
+```
+
+Run:
+```
+qemu-system-riscv64 -M spike -m 256M -nographic \
+	-kernel build/platform/spike/firmware/fw_payload.elf
+```
+
+**Linux Kernel Payload**
+
+Note: We assume that the Linux kernel is compiled using
+*arch/riscv/configs/defconfig*.
+
+Build:
+```
+make PLATFORM=spike FW_PAYLOAD_PATH=<linux_build_directory>/arch/riscv/boot/Image
+```
+
+Run:
+```
+qemu-system-riscv64 -M spike -m 256M -nographic \
+	-kernel build/platform/spike/firmware/fw_payload.elf \
+	-initrd <path_to_cpio_ramdisk> \
+	-append "root=/dev/ram rw console=hvc0 earlycon=sbi"
+```
+or
+```
+qemu-system-riscv64 -M spike -m 256M -nographic \
+	-bios build/platform/spike/firmware/fw_jump.elf \
+	-kernel <linux_build_directory>/arch/riscv/boot/Image \
+	-initrd <path_to_cpio_ramdisk> \
+	-append "root=/dev/ram rw console=hvc0 earlycon=sbi"
+```