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, 7 insertions, 2 deletions

diff --git a/bbl/bbl.ac b/bbl/bbl.ac
index 80d3b06..55ff50b 100644
--- a/bbl/bbl.ac
+++ b/bbl/bbl.ac
@@ -4,8 +4,13 @@ AS_IF([test "x$enable_logo" == "xyes"], [
 ])
 
 AC_ARG_WITH([payload], AS_HELP_STRING([--with-payload], [Set ELF payload for bbl]),
-  [AC_SUBST([BBL_PAYLOAD], $with_payload, [Kernel payload for bbl])],
-  [AC_SUBST([BBL_PAYLOAD], [dummy_payload], [Kernel payload for bbl])])
+  [
+   AC_SUBST([BBL_PAYLOAD], $with_payload, [Kernel payload for bbl])
+   AC_DEFINE(RELAXED_ALIGNMENT,[0],[Use relaxed payload alignment])
+  ], [
+   AC_SUBST([BBL_PAYLOAD], [dummy_payload], [Kernel payload for bbl])
+   AC_DEFINE(RELAXED_ALIGNMENT,[1],[Use relaxed payload alignment])
+  ])
 
 AC_ARG_WITH([logo], AS_HELP_STRING([--with-logo], [Specify a better logo]),
   [AC_SUBST([BBL_LOGO_FILE], $with_logo, [Logo for bbl])],