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.

2 files changed, 34 insertions, 3 deletions

diff --git a/arch/riscv64/init/init.c b/arch/riscv64/init/init.c
index 92f8053..278b9db 100644
--- a/arch/riscv64/init/init.c
+++ b/arch/riscv64/init/init.c
@@ -54,7 +54,8 @@ static void init_bootmem(uintptr_t load_addr, uintptr_t ram_base)
 
 	extern char *__kernel;
 	extern char *__kernel_size;
-	uintptr_t top = load_addr + (uintptr_t)&__kernel + (uintptr_t)&__kernel_size;
+	uintptr_t top = load_addr + (uintptr_t)&__kernel +
+	                (uintptr_t)&__kernel_size;
 
 	/* this could be risky, as we might overwrite some bits of initrd or fdt
 	 * if they're allocated too close to the kernel payload.
@@ -119,3 +120,17 @@ void init(void *fdt, pm_t load_addr)
 
 	jump_to_kernel(fdt, ram_base, (void *)VM_KERN);
 }
+
+#if GENERIC_UBOOT
+void init_go(int argc, char **argv, pm_t load_addr)
+{
+	if (argc != 2)
+		return;
+
+	void *fdt = (void *)strtouintptr(argv[1]);
+
+	/* fdt is passed as second argument, load address first but since we
+	 * already have it due to our ingenious _start, no need to parse it */
+	init(fdt, load_addr);
+}
+#endif
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