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, 5 insertions, 0 deletions

diff --git a/machine/fdt.h b/machine/fdt.h
index d436778..6c8a6fe 100644
--- a/machine/fdt.h
+++ b/machine/fdt.h
@@ -59,6 +59,7 @@ void query_mem(uintptr_t fdt);
 void query_harts(uintptr_t fdt);
 void query_plic(uintptr_t fdt);
 void query_clint(uintptr_t fdt);
+void query_chosen(uintptr_t fdt);
 
 // Remove information from FDT
 void filter_harts(uintptr_t fdt, long *disabled_hart_mask);
@@ -68,6 +69,10 @@ void filter_compat(uintptr_t fdt, const char *compat);
 // The hartids of available harts
 extern uint64_t hart_mask;
 
+// Optional FDT preloaded external payload
+extern void* kernel_start;
+extern void* kernel_end;
+
 #ifdef PK_PRINT_DEVICE_TREE
 // Prints the device tree to the console as a DTS
 void fdt_print(uintptr_t fdt);