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>

7 files changed, 156 insertions, 9 deletions

diff --git a/kernel/dt.c b/kernel/dt.c
new file mode 100644
index 0000000..55fa970
--- /dev/null
+++ b/kernel/dt.c
@@ -0,0 +1,6 @@
+#include <fbos/dt.h>
+
+__kernel void parse_dtb(uint64_t *dtb)
+{
+	__unused(dtb);
+}
diff --git a/kernel/fbos.ld.S b/kernel/fbos.ld.S
index f288161..f74d90f 100644
--- a/kernel/fbos.ld.S
+++ b/kernel/fbos.ld.S
@@ -3,15 +3,15 @@
 SECTIONS {
 	// Ensure that the image starts at the very exact address SBI expects it to.
 	. = LINK_ADDR;
+	_start = .;
+	. = ALIGN(PAGE_SIZE);
 
 	// You would usually want to separate the head section into its own thing
 	// instead of clumping it into the main `.text` one. Well, I'm no expert on
 	// linker configuration, so patches are welcome :)
 	.text : {
-		// The very first thing has to be the `_start` function, which is where
-		// SBI will jump into. Afterwards comes the rest of `.text.head`.
-		_start = .;
-		*(.text.head)
+		_text = .;
+		*(.head.text)
 
 		// Aligning it to a full page is maybe a bit too much considering how
 		// small `.text.head` really is. I just saw this same thing on the Linux
@@ -23,7 +23,7 @@ SECTIONS {
 		// that large. That is, we put first the very core of the kernel, and
 		// the rest can go wherever.
 		__kernel_text_start = .;
-		*(.text.kernel)
+		*(.kernel.text)
 		__kernel_text_end = .;
 
 		// And the rest.
@@ -49,4 +49,6 @@ SECTIONS {
 	.bss : {
     	*(.bss)
 	}
+
+	_end = .;
 }
diff --git a/kernel/head.S b/kernel/head.S
index ba155cb..de7852b 100644
--- a/kernel/head.S
+++ b/kernel/head.S
@@ -1,6 +1,55 @@
+#include <fbos/mm.h>
+
 .global _start
-.section .text.head
+.section .head.text
 
 _start:
-	// TODO
-	call start_kernel
+	// Mask all interrupts
+	csrw sie, zero
+	csrw sip, zero
+
+	// Flush the instruction cache
+	fence.i
+
+	// Reset all registers except ra, a0, a1.
+	li sp, 0
+	li gp, 0
+	li tp, 0
+	li t0, 0
+	li t1, 0
+	li t2, 0
+	li s0, 0
+	li s1, 0
+	li a2, 0
+	li a3, 0
+	li a4, 0
+	li a5, 0
+	li a6, 0
+	li a7, 0
+	li s2, 0
+	li s3, 0
+	li s4, 0
+	li s5, 0
+	li s6, 0
+	li s7, 0
+	li s8, 0
+	li s9, 0
+	li s10, 0
+	li s11, 0
+	li t3, 0
+	li t4, 0
+	li t5, 0
+	li t6, 0
+	csrw sscratch, 0
+
+	// Point tp and sp to the init task.
+	la tp, init_task
+	la sp, init_task + THREAD_SIZE
+
+	// The `start_kernel` function requires an argument to be passed, which is
+	// the pointer to the `fdt` blob. The bootloader puts this on the `a1`
+	// register, so let's move it now to `a0`.
+	mv a0, a1
+
+	// Start the kernel.
+	tail start_kernel
diff --git a/kernel/main.c b/kernel/main.c
index 325f858..07188b9 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -1,12 +1,28 @@
 #include <fbos/init.h>
+#include <fbos/printk.h>
+#include <fbos/mm.h>
+#include <fbos/sched.h>
+#include <fbos/dt.h>
+
+unsigned long init_stack[THREAD_SIZE / sizeof(unsigned long)];
+
+struct task_struct init_task = { .stack = init_stack };
 
 /*
  * This is the main entry point of the kernel after head.S is done. This
  * function can (and will) assume that everything has been reset and that we can
  * start the whole thing.
  */
-__noreturn __kernel void start_kernel(void)
+__noreturn __kernel void start_kernel(uintptr_t *dtb)
 {
+	// TODO: disable irqs, etc.
+
+	printk("Welcome to FizzBuzz OS!\n");
+
+	parse_dtb(dtb);
+
+	// TODO: reenable stuff
+
 	for (;;)
 		;
 }
diff --git a/kernel/printk.c b/kernel/printk.c
new file mode 100644
index 0000000..c0eedd9
--- /dev/null
+++ b/kernel/printk.c
@@ -0,0 +1,23 @@
+#include <fbos/printk.h>
+#include <fbos/string.h>
+#include <fbos/sbi.h>
+
+void __noreturn __kernel die(const char *const message)
+{
+	if (message) {
+		printk(message);
+	}
+
+	for (;;)
+		;
+}
+
+void __kernel printk(const char *const message)
+{
+	size_t len = strlen(message);
+	if (!len) {
+		return;
+	}
+
+	sbi_ecall2(DBCN_EXT, DBCN_WRITE, len, (unsigned long)message);
+}
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;
+}
diff --git a/kernel/strlen.S b/kernel/strlen.S
new file mode 100644
index 0000000..6185b94
--- /dev/null
+++ b/kernel/strlen.S
@@ -0,0 +1,22 @@
+.globl strlen
+.type strlen, @function
+
+/*
+ * Defined in include/fbos/string.h
+ *
+ *     size_t strlen(const char *str)
+ *
+ * Returns (a0): string length.
+ * Parameter (a0): string to measure.
+ * Clobbers: t0, t1.
+ */
+strlen:
+	mv t1, a0
+1:
+	lbu t0, 0(t1)
+	beqz t0, 2f
+	addi t1, t1, 1
+	j 1b
+2:
+	sub a0, t1, a0
+	ret