Hi,
On 3/13/25 4:45 PM, Denis Mukhin via B4 Relay wrote:
> 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.
Looks like unsigned long to me.
> + * @keep: Keep after the real driver is available.
These 4 "parameters" are not the function arguments.
@s is the function argument.
You could say in a comment that @s is scanned to obtain
these 4 parameters or something like that.
> + */
> +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);
>
> ---
--
~Randy
