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

[Denis Mukhin]

On Thursday, March 13th, 2025 at 5:04 PM, Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:

> 
> 
> 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.

Sorry, I messed the description.

These 4 parameters meant to describe the arguments of the string to parse:
   membase[,{nocfg|baudrate}][,keep]

I will update.

Thanks!

> 
> > + */
> > +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