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, 57 insertions, 0 deletions

diff --git a/machine/fdt.c b/machine/fdt.c
index 061b19e..27f5462 100644
--- a/machine/fdt.c
+++ b/machine/fdt.c
@@ -548,6 +548,63 @@ void filter_compat(uintptr_t fdt, const char *compat)
   fdt_scan(fdt, &cb);
 }
 
+//////////////////////////////////////////// CHOSEN SCAN ////////////////////////////////////////
+
+struct chosen_scan {
+  const struct fdt_scan_node *chosen;
+  void* kernel_start;
+  void* kernel_end;
+};
+
+static void chosen_open(const struct fdt_scan_node *node, void *extra)
+{
+  struct chosen_scan *scan = (struct chosen_scan *)extra;
+  if (!strcmp(node->name, "chosen")) {
+    scan->chosen = node;
+  }
+}
+
+static int chosen_close(const struct fdt_scan_node *node, void *extra)
+{
+  struct chosen_scan *scan = (struct chosen_scan *)extra;
+  if (scan->chosen && scan->chosen == node) {
+    scan->chosen = NULL;
+  }
+  return 0;
+}
+
+static void chosen_prop(const struct fdt_scan_prop *prop, void *extra)
+{
+  struct chosen_scan *scan = (struct chosen_scan *)extra;
+  uint64_t val;
+  if (!scan->chosen) return;
+  if (!strcmp(prop->name, "riscv,kernel-start")) {
+    fdt_get_address(prop->node->parent, prop->value, &val);
+    scan->kernel_start = (void*)val;
+  } else if (!strcmp(prop->name, "riscv,kernel-end")) {
+    fdt_get_address(prop->node->parent, prop->value, &val);
+    scan->kernel_end = (void*)val;
+  }
+}
+
+void query_chosen(uintptr_t fdt)
+{
+  struct fdt_cb cb;
+  struct chosen_scan chosen;
+
+  memset(&cb, 0, sizeof(cb));
+  cb.open = chosen_open;
+  cb.close = chosen_close;
+  cb.prop = chosen_prop;
+
+  memset(&chosen, 0, sizeof(chosen));
+  cb.extra = &chosen;
+
+  fdt_scan(fdt, &cb);
+  kernel_start = chosen.kernel_start;
+  kernel_end = chosen.kernel_end;
+}
+
 //////////////////////////////////////////// HART FILTER ////////////////////////////////////////
 
 struct hart_filter {