start trying to boot on visionfive 2

+ Not bootable quite yet. Among other things, I couldn't
  get the current starfive u-boot fork to boot, so
  try adding support for booting with precompiled u-boot
  via the `go` command. Initial testing with qemu shows that
  this should be possible, and if it works, might be useful
  in the (far) future with other slightly janky SBCs.

  Also, NS16550 is 8250-based, and I'm really only using the base
  8250, so rename and add visionfive 2 uart to list of compatibles.
  Visionfive 2 is still completely untested.

1 files changed, 18 insertions, 2 deletions

diff --git a/arch/riscv64/init/start.S b/arch/riscv64/init/start.S
index 7439d93..b6394f7 100644
--- a/arch/riscv64/init/start.S
+++ b/arch/riscv64/init/start.S
@@ -1,17 +1,33 @@
 /* SPDX-License-Identifier: copyleft-next-0.3.1 */
 /* Copyright 2021 - 2022, Kim Kuparinen < kimi.h.kuparinen@gmail.com > */
 
+/* generic u-boot passes argc and argv as if we were a regular
+ * program, so our load address must be placed third.
+ * If we have a 'custom' u-boot with kmi support, the fdt
+ * will be passed to us directly, so the load address can go second.
+ */
+#if GENERIC_UBOOT
+# define LOAD_REG a2
+# define INIT init_go
+#else
+# define LOAD_REG a1
+# define INIT init
+#endif
+
 .section .init
 .global _start
 /* Entry point to the kernel loader. */
 _start:
 /* get load address */
-auipc a1, 0
+auipc LOAD_REG, 0
 /* keep using bootloader stack for now */
 //li sp, PM_STACK_TOP
 /* make sure there's no garbage in tp, important for id assignment */
 li tp, 0
-call init
+call INIT
+// INIT shouldn't return, but if we do we might as well just jump back to u-boot
+// or wherever
+ret
 
 .section .text
 .global jump_to_kernel