Merge patch series "serial: goldfish: Add debug uart support"

This series from Daniel Palmer <daniel@0x0f.com> improves debug UART
support on QEMU on M68K by adding debug uart support to the serial
driver.

Link: https://lore.kernel.org/r/20260309105110.672832-1-daniel@0x0f.com

arch/m68k/cpu/m680x0/start.S

@@ -42,6 +42,11 @@ ENTRY(_start)
 	/* Setup initial stack pointer */
 	move.l	#CFG_SYS_INIT_SP_ADDR, %sp
 
+	/* Setup the debug uart if enabled */
+#ifdef CONFIG_DEBUG_UART
+	bsr.l	debug_uart_init
+#endif
+
 	/*
 	 * Allocate Global Data (GD)
 	 * board_init_f_alloc_reserve(top) returns the new top of stack in %d0

doc/board/emulation/qemu-m68k.rst

@@ -31,6 +31,16 @@ The minimal QEMU command line to get U-Boot up and running is:
 Note that the `-nographic` option is used to redirect the console to stdio,
 which connects to the emulated Goldfish TTY device.
 
+Debugging U-Boot
+----------------
+
+If you need early debugging output enable `CONFIG_DEBUG_UART_GOLDFISH`.
+The base address for the UART can be found by activating the QEMU monitor,
+running `info qtree`, and then looking for the goldfish tty device and
+taking the mmio address.
+
+Baud rate doesn't matter.
+
 Hardware Support
 ----------------
 The following QEMU virt peripherals are supported in U-Boot:

drivers/serial/Kconfig

@@ -510,6 +510,14 @@ config DEBUG_UART_XTENSA_SEMIHOSTING
 	  start up driver model. The driver will be available until the real
 	  driver model serial is running.
 
+config DEBUG_UART_GOLDFISH
+	bool "Goldfish TTY"
+	help
+	  Select this to enable the debug UART using the Goldfish TTY driver.
+	  This provides basic serial output from the console without needing to
+	  start up driver model. The driver will be available until the real
+	  driver model serial is running.
+
 endchoice
 
 config DEBUG_UART_BASE

drivers/serial/serial_goldfish.c

@@ -115,3 +115,21 @@ U_BOOT_DRIVER(serial_goldfish) = {
 	.priv_auto = sizeof(struct goldfish_tty_priv),
 	.flags	= DM_FLAG_PRE_RELOC,
 };
+
+#ifdef CONFIG_DEBUG_UART_GOLDFISH
+
+#include <debug_uart.h>
+
+static inline void _debug_uart_init(void)
+{
+}
+
+static inline void _debug_uart_putc(int ch)
+{
+	void __iomem *base = (void __iomem *)CONFIG_VAL(DEBUG_UART_BASE);
+
+	__raw_writel(ch, base + GOLDFISH_TTY_PUT_CHAR);
+}
+
+DEBUG_UART_FUNCS
+#endif