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

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Message-ID: <QKiXvDxnTzqiTDN61eP_MtKY53BNMuH5wG_PHWrr_VbdjypCjsDmioJ7SYg4a8sfp1In8OV0aihcLffmjkNCklhe5cTZ3km22cS604TpIYE=@proton.me>
Date: Fri, 14 Mar 2025 00:35:51 +0000
From: Denis Mukhin <dmkhn@...ton.me>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/early_printk: add MMIO-based UARTs

On Thursday, March 13th, 2025 at 5:04 PM, Dave Hansen <dave.hansen@...el.com> wrote:

> 
> 
> 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?

Correct, this is not off the shelf laptop.

> 
> Is there a driver for it during normal runtime?

Yes, that is a variant of NS16550 UART.

> 
> > 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.

Thanks!

--
Denis