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Message-Id: <20250324-earlyprintk-v3-1-aee7421dc469@ford.com>
Date: Mon, 24 Mar 2025 17:55:40 -0700
From: Denis Mukhin via B4 Relay <devnull+dmukhin.ford.com@...nel.org>
To: 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,
Denis Mukhin <dmukhin@...d.com>
Subject: [PATCH v3] x86/early_printk: add MMIO-based UARTs
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
Also, update kernel parameters documentation with the new syntax and
add missing 'nocfg' setting to PCI serial cards description.
Signed-off-by: Denis Mukhin <dmukhin@...d.com>
---
Changes in v3:
- Fixed formatting in early_mmio_serial_init()
- Rebased on tip/master, new code switched to using static_call_update()
- Doc update: added nocfg description for earlyprintk=pciserial and a note
on 32-bit memory-mapped UARTs
- Link to v2: https://lore.kernel.org/r/20250314-earlyprintk-v2-1-2bcbe05290b8@ford.com
Changes in v2:
- Fixed description of early_mmio_serial_init()
- Link to v1: https://lore.kernel.org/r/20250313-earlyprintk-v1-1-8f818d77a8dd@ford.com
---
Documentation/admin-guide/kernel-parameters.txt | 9 ++++-
arch/x86/kernel/early_printk.c | 45 ++++++++++++++++++++++++-
2 files changed, 52 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3e5e41cbe3ce907bb286a6d86456241e02976f6d..1d1eaeac04b31ba808b9196d0f928050acfd17b7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1407,14 +1407,21 @@
earlyprintk=serial[,0x...[,baudrate]]
earlyprintk=ttySn[,baudrate]
earlyprintk=dbgp[debugController#]
- earlyprintk=pciserial[,force],bus:device.function[,baudrate]
+ earlyprintk=pciserial[,force],bus:device.function[,{nocfg|baudrate}]
earlyprintk=xdbc[xhciController#]
earlyprintk=bios
+ earlyprintk=mmio,membase[,{nocfg|baudrate}]
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.
+ Only 32-bit memory addresses are supported for "mmio"
+ and "pciserial" devices.
+
+ 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 fc1714bad04588abc3468a0e72df26513250162b..611f27e3890c2852cece31a9e948e7212c4e10fe 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -190,7 +190,6 @@ static __init void early_serial_init(char *s)
early_serial_hw_init(divisor);
}
-#ifdef CONFIG_PCI
static __noendbr void mem32_serial_out(unsigned long addr, int offset, int value)
{
u32 __iomem *vaddr = (u32 __iomem *)addr;
@@ -207,6 +206,45 @@ static __noendbr unsigned int mem32_serial_in(unsigned long addr, int offset)
}
ANNOTATE_NOENDBR_SYM(mem32_serial_in);
+/*
+ * early_mmio_serial_init() - Initialize MMIO-based early serial console.
+ * @s: MMIO-based serial specification.
+ */
+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)) {
+ /* NB: only 32-bit addresses are supported. */
+ membase = simple_strtoul(s, &e, 16);
+ early_serial_base = (unsigned long)early_ioremap(membase, PAGE_SIZE);
+
+ static_call_update(serial_in, mem32_serial_in);
+ static_call_update(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()
*
@@ -351,6 +389,11 @@ 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: cc086afdd3132fa2a80c3891639c2352521e5d73
change-id: 20250313-earlyprintk-f68bcf10febc
Best regards,
--
Denis Mukhin <dmukhin@...d.com>
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