diff options
| author | Denis Mukhin <dmukhin@ford.com> | 2025-03-24 17:55:40 -0700 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2025-03-25 08:35:38 +0100 |
| commit | 3181424aeac2f6596534bf43021a10eae294a9b0 (patch) | |
| tree | 20bf2ffea4a53c2f2a8334c2384eb9021f42d045 /Documentation/admin-guide/kernel-parameters.txt | |
| parent | 2c118f50d7fd4d9aefc4533a26f83338b2906b7a (diff) | |
| download | linux-next-3181424aeac2f6596534bf43021a10eae294a9b0.tar.gz linux-next-3181424aeac2f6596534bf43021a10eae294a9b0.zip | |
x86/early_printk: Add support for MMIO-based UARTs
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.
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 the missing 'nocfg' setting to the PCI serial cards description.
Signed-off-by: Denis Mukhin <dmukhin@ford.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250324-earlyprintk-v3-1-aee7421dc469@ford.com
Diffstat (limited to 'Documentation/admin-guide/kernel-parameters.txt')
| -rw-r--r-- | Documentation/admin-guide/kernel-parameters.txt | 9 |
1 files changed, 8 insertions, 1 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 866427da6add..649261e97628 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. |