[PATCH v4 3/3] docs/system: riscv: Add pflash usage details

[Sunil V L]

pflash devices can be used in virt machine for different
purposes like for ROM code or S-mode FW payload. Add a
section in the documentation on how to use pflash devices
for different purposes.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
---
 docs/system/riscv/virt.rst | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/docs/system/riscv/virt.rst b/docs/system/riscv/virt.rst
index 4b16e41d7f..c3e0b7a9c3 100644
--- a/docs/system/riscv/virt.rst
+++ b/docs/system/riscv/virt.rst
@@ -53,6 +53,35 @@ with the default OpenSBI firmware image as the -bios. It 
also supports
 the recommended RISC-V bootflow: U-Boot SPL (M-mode) loads OpenSBI fw_dynamic
 firmware and U-Boot proper (S-mode), using the standard -bios functionality.
 
+Using flash devices
+-------------------
+
+The first flash device (pflash0) can contain either ROM code
+or S-mode payload firmware code. If the pflash0 contains the
+ROM code, -bios should be set to none. If -bios is not set to
+none, pflash0 is assumed to contain S-mode payload code.
+
+Firmware images used for pflash should be of size 32 MiB.
+
+To boot as ROM code:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -bios none \
+     -blockdev node-name=pflash0,driver=file,filename=<rom_code> \
+     -M virt,pflash0=pflash0 \
+     ... other args ....
+
+To boot as read-only S-mode payload:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 \
+     -blockdev 
node-name=pflash0,driver=file,read-only=on,filename=<s-mode_fw_code> \
+     -blockdev node-name=pflash1,driver=file,filename=<s-mode_fw_vars> \
+     -M virt,pflash0=pflash0,pflash1=pflash1 \
+     ... other args ....
+
 Machine-specific options
 ------------------------
 
-- 
2.34.1