Setup the stack for an init task

Bootstrap an init task which holds at least the initial stack that is to
be used when setting up the registers (such as `sp` and `tp`).

In order to guarantee that the stack and the rest of the registers are
set up correctly, this commit also provides a raw implementation of a
`printk` function.

Moreover, this commit also adds an argument that must be passed to
`start_kernel`, which is the pointer to the embedded `fdt` blob. This
argument will be used by later work so to fetch, at least, the base
address for the initial ram disk. This was also used to test that the
stack was working as expected.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/kernel/sbi.c b/kernel/sbi.c
new file mode 100644
index 0000000..ff7556c
--- /dev/null
+++ b/kernel/sbi.c
@@ -0,0 +1,29 @@
+#include <fbos/compiler.h>
+#include <fbos/sbi.h>
+
+/* Implementation taken from the Linux kernel (6.12) */
+__kernel struct sbi_ret __sbi_ecall(unsigned long arg0, unsigned long arg1, unsigned long arg2,
+									unsigned long arg3, unsigned long arg4, unsigned long arg5,
+									int fid, int ext)
+{
+	struct sbi_ret ret;
+
+	register uintptr_t a0 asm("a0") = (uintptr_t)(arg0);
+	register uintptr_t a1 asm("a1") = (uintptr_t)(arg1);
+	register uintptr_t a2 asm("a2") = (uintptr_t)(arg2);
+	register uintptr_t a3 asm("a3") = (uintptr_t)(arg3);
+	register uintptr_t a4 asm("a4") = (uintptr_t)(arg4);
+	register uintptr_t a5 asm("a5") = (uintptr_t)(arg5);
+	register uintptr_t a6 asm("a6") = (uintptr_t)(fid);
+	register uintptr_t a7 asm("a7") = (uintptr_t)(ext);
+
+	asm volatile("ecall"
+				 : "+r"(a0), "+r"(a1)
+				 : "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(a6), "r"(a7)
+				 : "memory");
+
+	ret.error = (long)a0;
+	ret.value = (long)a1;
+
+	return ret;
+}