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>
