linux-kernel - Re: [PATCH] x86/early_printk: add MMIO-based UARTs

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Message-ID: <60067986-ce08-45da-85ba-a9ea8fedfe06@infradead.org>
Date: Thu, 13 Mar 2025 17:04:50 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: dmukhin@...d.com, Jonathan Corbet <corbet@....net>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/early_printk: add MMIO-based UARTs

Hi,

On 3/13/25 4:45 PM, Denis Mukhin via B4 Relay wrote:
> From: Denis Mukhin <dmukhin@...d.com>
> 
> 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@...d.com>
> ---
>  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