From df3bb750a722cf6155bee151c61fa0af9335d898 Mon Sep 17 00:00:00 2001
From: Mark Poliakov <mark@alnyan.me>
Date: Wed, 31 Mar 2021 00:19:32 +0300
Subject: [PATCH] Initial commit

---
 .gitignore |  2 ++
 Makefile   | 18 ++++++++++++++++++
 entry.S    | 15 +++++++++++++++
 kernel.c   | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 link.ld    | 20 ++++++++++++++++++++
 5 files changed, 101 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 Makefile
 create mode 100644 entry.S
 create mode 100644 kernel.c
 create mode 100644 link.ld

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..009c450
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/kernel.elf
+/kernel.bin
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..cbc7f8b
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,18 @@
+CFLAGS=-ffreestanding \
+	   -O0
+LDFLAGS=-nostdlib
+
+all: kernel.bin
+
+kernel.bin: kernel.elf
+	$(CROSS_COMPILE)objcopy -O binary $< $@
+
+kernel.elf: entry.S kernel.c link.ld
+	$(CROSS_COMPILE)gcc $(CFLAGS) $(LDFLAGS) \
+		-o $@ \
+		-Tlink.ld \
+		entry.S \
+		kernel.c
+
+clean:
+	rm -f kernel.elf kernel.bin
diff --git a/entry.S b/entry.S
new file mode 100644
index 0000000..238db67
--- /dev/null
+++ b/entry.S
@@ -0,0 +1,15 @@
+.section .text.boot
+.global _entry
+.type _entry, %function
+_entry:
+    adr x1, bsp_stack_top
+    mov sp, x1
+
+    bl kernel_main
+    b .
+.size _entry, . - _entry
+
+.section .bss
+bsp_stack_bottom:
+    .skip 65536
+bsp_stack_top:
diff --git a/kernel.c b/kernel.c
new file mode 100644
index 0000000..929ef6b
--- /dev/null
+++ b/kernel.c
@@ -0,0 +1,46 @@
+#include <stdint.h>
+
+#define UART0_BASE      0x05000000
+#define R_WDOG_BASE     0x07020400
+
+#define WDOG_CTRL_KEY       (0xA57 << 1)
+#define WDOG_CTRL_RESTART   (1 << 0)
+#define WDOG_CFG_SYS        (1 << 0)
+#define WDOG_MODE_EN        (1 << 0)
+
+struct watchdog {
+    uint32_t irq_en;
+    uint32_t irq_sta;
+    uint32_t res[2];
+    uint32_t ctrl;
+    uint32_t cfg;
+    uint32_t mode;
+};
+
+static volatile struct watchdog *const r_wdog = (struct watchdog *) R_WDOG_BASE;
+
+static inline void delay(uint32_t t) {
+    for (uint32_t i = 0; i < t; ++i) {
+        asm volatile ("nop");
+    }
+}
+
+static void uart_tx(uint8_t byte) {
+    *((uint8_t *) UART0_BASE) = byte;
+}
+
+static _Noreturn void board_reset(void) {
+    r_wdog->cfg = WDOG_CFG_SYS;
+    r_wdog->mode = WDOG_MODE_EN;
+    r_wdog->ctrl = WDOG_CTRL_KEY | WDOG_CTRL_RESTART;
+    while (1);
+}
+
+_Noreturn void kernel_main(uintptr_t dt_phys_addr) {
+    for (const char *p = "It's alive\r\n"; *p; ++p) {
+        uart_tx(*p);
+        delay(1000);
+    }
+
+    board_reset();
+}
diff --git a/link.ld b/link.ld
new file mode 100644
index 0000000..fa7b6ba
--- /dev/null
+++ b/link.ld
@@ -0,0 +1,20 @@
+ENTRY(_entry);
+
+MEMORY {
+    ram : ORIGIN = 0x42000000, LENGTH = 992M
+}
+
+SECTIONS {
+    .text : ALIGN(4K) {
+        *(.text.boot)
+        *(.text*)
+    }>ram
+
+    .data : ALIGN(4K) {
+        *(.data*)
+    }>ram
+
+    .bss : ALIGN(4K) {
+        *(.bss*)
+    }>ram
+}