x86/early_printk: Add support for MMIO-based UARTs

During the bring-up of an x86 board, the kernel was crashing before
reaching the platform's console driver because of a bug in the firmware,
leaving no trace of the boot progress.

The only available method to debug the kernel boot process was via the
platform's MMIO-based UART, as the board lacked an I/O port-based UART,
PCI UART, or functional video output.

Then it turned out that earlyprintk= does not have a knob to configure
the MMIO-mapped UART.

Extend the early printk facility to support platform MMIO-based UARTs
on x86 systems, enabling debugging during the system bring-up phase.

The command line syntax to enable platform MMIO-based UART is:

  earlyprintk=mmio,membase[,{nocfg|baudrate}][,keep]

Note, the change does not integrate MMIO-based UART support to:

  arch/x86/boot/early_serial_console.c

Also, update kernel parameters documentation with the new syntax and
add the missing 'nocfg' setting to the PCI serial cards description.

Signed-off-by: Denis Mukhin <dmukhin@ford.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250324-earlyprintk-v3-1-aee7421dc469@ford.com

Documentation/admin-guide/kernel-parameters.txt

@@ -1407,14 +1407,21 @@
 			earlyprintk=serial[,0x...[,baudrate]]
 			earlyprintk=ttySn[,baudrate]
 			earlyprintk=dbgp[debugController#]
-			earlyprintk=pciserial[,force],bus:device.function[,baudrate]
+			earlyprintk=pciserial[,force],bus:device.function[,{nocfg|baudrate}]
 			earlyprintk=xdbc[xhciController#]
 			earlyprintk=bios
+			earlyprintk=mmio,membase[,{nocfg|baudrate}]
 
 			earlyprintk is useful when the kernel crashes before
 			the normal console is initialized. It is not enabled by
 			default because it has some cosmetic problems.
 
+			Only 32-bit memory addresses are supported for "mmio"
+			and "pciserial" devices.
+
+			Use "nocfg" to skip UART configuration, assume
+			BIOS/firmware has configured UART correctly.
+
 			Append ",keep" to not disable it when the real console
 			takes over.
 

arch/x86/kernel/early_printk.c

@@ -190,7 +190,6 @@ static __init void early_serial_init(char *s)
 	early_serial_hw_init(divisor);
 }
 
-#ifdef CONFIG_PCI
 static __noendbr void mem32_serial_out(unsigned long addr, int offset, int value)
 {
 	u32 __iomem *vaddr = (u32 __iomem *)addr;
@@ -207,6 +206,45 @@ static __noendbr unsigned int mem32_serial_in(unsigned long addr, int offset)
 }
 ANNOTATE_NOENDBR_SYM(mem32_serial_in);
 
+/*
+ * early_mmio_serial_init() - Initialize MMIO-based early serial console.
+ * @s: MMIO-based serial specification.
+ */
+static __init void early_mmio_serial_init(char *s)
+{
+	unsigned long baudrate;
+	unsigned long membase;
+	char *e;
+
+	if (*s == ',')
+		s++;
+
+	if (!strncmp(s, "0x", 2)) {
+		/* NB: only 32-bit addresses are supported. */
+		membase = simple_strtoul(s, &e, 16);
+		early_serial_base = (unsigned long)early_ioremap(membase, PAGE_SIZE);
+
+		static_call_update(serial_in, mem32_serial_in);
+		static_call_update(serial_out, mem32_serial_out);
+
+		s += strcspn(s, ",");
+		if (*s == ',')
+			s++;
+	}
+
+	if (!strncmp(s, "nocfg", 5)) {
+		baudrate = 0;
+	} else {
+		baudrate = simple_strtoul(s, &e, 0);
+		if (baudrate == 0 || s == e)
+			baudrate = DEFAULT_BAUD;
+	}
+
+	if (baudrate)
+		early_serial_hw_init(115200 / baudrate);
+}
+
+#ifdef CONFIG_PCI
 /*
  * early_pci_serial_init()
  *
@@ -351,6 +389,11 @@ static int __init setup_early_printk(char *buf)
 	keep = (strstr(buf, "keep") != NULL);
 
 	while (*buf != '\0') {
+		if (!strncmp(buf, "mmio", 4)) {
+			early_mmio_serial_init(buf + 4);
+			early_console_register(&early_serial_console, keep);
+			buf += 4;
+		}
 		if (!strncmp(buf, "serial", 6)) {
 			buf += 6;
 			early_serial_init(buf);