From: Denis Mukhin <dmukhin@xxxxxxxx> 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. It was discovered that 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 Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx> --- Documentation/admin-guide/kernel-parameters.txt | 4 +++ arch/x86/kernel/early_printk.c | 45 ++++++++++++++++++++++++- 2 files changed, 48 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index fb8752b42ec8582b8750d7e014c4d76166fa2fc1..bee9ee18a506d019dc3d330268e3e1c83434ebba 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -1414,11 +1414,15 @@ earlyprintk=pciserial[,force],bus:device.function[,baudrate] earlyprintk=xdbc[xhciController#] earlyprintk=bios + earlyprintk=mmio,membase[,{nocfg|baudrate}][,keep] 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. + 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. diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c index 44f937015e1e25bf41532eb7e1031a6be32a6523..19248c73b5b0950e9edf1a60ba67829f1cd3279e 100644 --- a/arch/x86/kernel/early_printk.c +++ b/arch/x86/kernel/early_printk.c @@ -191,7 +191,6 @@ static __init void early_serial_init(char *s) early_serial_hw_init(divisor); } -#ifdef CONFIG_PCI static void mem32_serial_out(unsigned long addr, int offset, int value) { u32 __iomem *vaddr = (u32 __iomem *)addr; @@ -206,6 +205,45 @@ static unsigned int mem32_serial_in(unsigned long addr, int offset) return readl(vaddr + offset); } +/* + * early_mmio_serial_init() - Initialize MMIO-based early serial console. + * @membase: UART base address. + * @nocfg: Skip configuration, assume BIOS has configured UART correctly. + * @baudrate (int): Baud rate. + * @keep: Keep after the real driver is available. + */ +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)) { + membase = simple_strtoul(s, &e, 16); + early_serial_base = (unsigned long)early_ioremap(membase, PAGE_SIZE); + serial_in = mem32_serial_in; + 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() * @@ -352,6 +390,11 @@ 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); --- base-commit: 8aed61b8334e00f4fe5de9f2df1cd183dc328a9d change-id: 20250313-earlyprintk-f68bcf10febc Best regards, -- Denis Mukhin <dmukhin@xxxxxxxx>