Commit 3181424a authored by Denis Mukhin's avatar Denis Mukhin Committed by Ingo Molnar
Browse files

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: default avatarDenis Mukhin <dmukhin@ford.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250324-earlyprintk-v3-1-aee7421dc469@ford.com
parent 2c118f50

Documentation/admin-guide/kernel-parameters.txt

+8 −1
Original line number Diff line number Diff line
@@ -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

+44 −1
Original line number Diff line number Diff line
@@ -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);