x86: support very early printk() if desired

Adapted from similar code in the x86_64 port.
Useful when debugging boot problems on actual x86
hardware if a JTAG isn't handy or feasible.

Turn this on for qemu_x86.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

arch/x86/Kconfig

@@ -108,6 +108,17 @@ endif # X86_MULTIBOOT_FRAMEBUF
 
 endif # X86_MULTIBOOT
 
+config X86_VERY_EARLY_CONSOLE
+	bool "Support very early boot printk"
+	depends on PRINTK
+	help
+	  Non-emulated X86 devices often require special hardware to attach
+	  a debugger, which may not be easily available. This option adds a
+	  very minimal serial driver which gets initialized at the very
+	  beginning of z_cstart(), via kernel_arch_init(). This driver enables
+	  printk to emit messages to the 16550 UART port 0 instance in device
+	  tree. This mini-driver assumes I/O to the UART is done via ports.
+
 source "arch/x86/core/Kconfig.ia32"
 source "arch/x86/core/Kconfig.x64"
 

arch/x86/core/CMakeLists.txt

@@ -12,6 +12,7 @@ zephyr_compile_options_ifdef(CONFIG_COVERAGE_GCOV
 zephyr_library_sources_if_kconfig(pcie.c)
 zephyr_library_sources_if_kconfig(reboot_rst_cnt.c)
 zephyr_library_sources_ifdef(CONFIG_X86_MULTIBOOT multiboot.c)
+zephyr_library_sources_ifdef(CONFIG_X86_VERY_EARLY_CONSOLE early_serial.c)
 
 if(CONFIG_X86_LONGMODE)
   include(x64.cmake)

arch/x86/core/early_serial.c

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2018 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <generated_dts_board.h>
+#include <arch/cpu.h>
+#include <misc/util.h>
+
+/* Super-primitive 8250/16550 serial output-only driver, 115200 8n1 */
+
+#define PORT ((io_port_t)DT_UART_NS16550_PORT_0_BASE_ADDR)
+
+#define REG_IER 0x01	/* Interrupt enable reg.          */
+#define REG_LCR 0x03	/* Line control reg.              */
+#define REG_MCR 0x04	/* Modem control reg.             */
+#define REG_LSR 0x05	/* Line status reg.               */
+#define REG_DL_LO 0x00	/* Divisor latch low byte	  */
+#define REG_DL_HI 0x01	/* Divisor latch high byte	  */
+
+#define IER_DISABLE		0x00
+#define LCR_8N1			(BIT(0) | BIT(1))
+#define LCR_DLAB_SELECT		BIT(7)
+#define MCR_DTR			BIT(0)
+#define MCR_RTS			BIT(1)
+#define LCR_THRE		BIT(5)
+
+static void serout(int c)
+{
+	while (!(sys_in8(PORT + REG_LSR) & LCR_THRE)) {
+	}
+	sys_out8(c, PORT);
+}
+
+static int console_out(int c)
+{
+	if (c == '\n') {
+		serout('\r');
+	}
+	serout(c);
+	return c;
+}
+
+extern void __printk_hook_install(int (*fn)(int));
+
+void z_x86_early_serial_init(void)
+{
+	/* In fact Qemu already has most of this set up and works by
+	 * default
+	 */
+	sys_out8(IER_DISABLE, PORT + REG_IER);		/* Disable interrupts */
+	sys_out8(LCR_DLAB_SELECT, PORT + REG_LCR);	/* DLAB select */
+	sys_out8(1, PORT + REG_DL_LO);			/* Baud divisor = 1 */
+	sys_out8(0, PORT + REG_DL_HI);
+	sys_out8(LCR_8N1, PORT + REG_LCR);	/* LCR = 8n1 + DLAB off */
+	sys_out8(MCR_DTR | MCR_RTS, PORT + REG_MCR);
+
+	/* Will be replaced later when a real serial driver comes up */
+	__printk_hook_install(console_out);
+}

arch/x86/include/ia32/kernel_arch_func.h

@@ -23,6 +23,11 @@ extern "C" {
 
 extern K_THREAD_STACK_DEFINE(_interrupt_stack, CONFIG_ISR_STACK_SIZE);
 
+#ifdef CONFIG_X86_VERY_EARLY_CONSOLE
+/* Setup ultra-minimal serial driver for printk() */
+void z_x86_early_serial_init(void);
+#endif
+
 /**
  *
  * @brief Performs architecture-specific initialization
@@ -38,6 +43,10 @@ static inline void kernel_arch_init(void)
 	_kernel.nested = 0;
 	_kernel.irq_stack = Z_THREAD_STACK_BUFFER(_interrupt_stack) +
 				CONFIG_ISR_STACK_SIZE;
+
+#ifdef CONFIG_X86_VERY_EARLY_CONSOLE
+	z_x86_early_serial_init();
+#endif
 #if CONFIG_X86_STACK_PROTECTION
 	z_x86_mmu_set_flags(&z_x86_kernel_pdpt, _interrupt_stack, MMU_PAGE_SIZE,
 			   MMU_ENTRY_NOT_PRESENT, MMU_PTE_P_MASK);

boards/x86/qemu_x86/qemu_x86_defconfig

@@ -18,3 +18,4 @@ CONFIG_X86_MMU=y
 CONFIG_DEBUG_INFO=y
 CONFIG_SCHED_SCALABLE=y
 CONFIG_WAITQ_SCALABLE=y
+CONFIG_X86_VERY_EARLY_CONSOLE=y