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

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Message-ID: <cfc6916e-802f-4727-aa74-b052f94d0101@intel.com>
Date: Thu, 13 Mar 2025 17:04:12 -0700
From: Dave Hansen <dave.hansen@...el.com>
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

On 3/13/25 16:45, Denis Mukhin via B4 Relay wrote:
> 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.

This is a pretty exotic piece of hardware, right? It's not some off the
shelf laptop?

Is there a driver for it during normal runtime?

> 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]

I'll stick this in the queue to take a closer look after the next merge
window closes. It's a bit on the late side in the 6.14 cycle for new stuff.

I do appreciate the importance of having this tool in your toolbox.
earlyprintk has saved my bacon more times than I can count.