RISC-V: Support separate firmware and kernel payload

Support for separate firmware and kernel payload is added
by updating BBL to read optional preloaded kernel address
attributes from device-tree using a similar mechanism to
that used to pass init ramdisk addresses to linux kernel.

    chosen {
        riscv,kernel-start = <0x00000000 0x80200000>;
        riscv,kernel-end = <0x00000000 0x80590634>;
    };

These attributes are added by QEMU and read by BBL when combining
-bios <firmware-image> and -kernel <kernel-image> options. e.g.

$ qemu-system-riscv64 -machine virt -bios bbl -kernel vmlinux

With this change, bbl can be compiled without --with-payload
and the dummy payload alignment is altered to make the memory
footprint of the firmware-only bbl smaller. The dummy payload
message is updated to indicate the alternative load method.

This load method could also be supported by a first stage boot
loader that reads seperate firmware and kernel from SPI flash.
The main advantage of this new mechanism is that it eases kernel
development by avoiding the riscv-pk packaging step after kernel
builds, makes building per repository artefacts for CI simpler,
and mimics bootloaders on other platforms that can load a kernel
image file directly. Ultimately BBL should use an SPI driver to
load the kernel image however this mechanism supports use cases
such such as QEMU's -bios, -kernel and -initrd options following
examples from other platforms that pass kernel entry to firmware
via device-tree.

Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Michael Clark <mjc@sifive.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/dummy_payload/dummy_entry.S b/dummy_payload/dummy_entry.S
index 92d4652..2473d8c 100644
--- a/dummy_payload/dummy_entry.S
+++ b/dummy_payload/dummy_entry.S
@@ -18,5 +18,11 @@ _start:
 
   .data
 str:
-  .asciz "This is bbl's dummy_payload.  To boot a real kernel, reconfigure\n\
-bbl with the flag --with-payload=PATH, then rebuild bbl.\n"
+  .asciz "This is bbl's dummy_payload.  To boot a real kernel, reconfigure bbl
+with the flag --with-payload=PATH, then rebuild bbl. Alternatively,
+bbl can be used in firmware-only mode by adding device-tree nodes
+for an external payload and use QEMU's -bios and -kernel options.\n
+    chosen {
+        riscv,kernel-start = <payload_start>;
+        riscv,kernel-end = <payload_end>;
+    };\n\n"