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, 0 deletions

diff --git a/bbl/payload.S b/bbl/payload.S
index 6a175aa..b6797aa 100644
--- a/bbl/payload.S
+++ b/bbl/payload.S
@@ -1,7 +1,15 @@
+#include "config.h"
 #include "encoding.h"
 
   .section ".payload","a",@progbits
+
+#if RELAXED_ALIGNMENT
+  /* align payload minimally */
+  .align 3
+#else
+  /* align payload to megapage */
   .align RISCV_PGSHIFT + RISCV_PGLEVEL_BITS
+#endif
 
   .globl _payload_start, _payload_end
 _payload_start: