linux-kernel - Re: [PATCH v3] x86/early_printk: add MMIO-based UARTs

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Message-ID: <vCbvBjUt9kPZhYP0tXAoVKrIn5hk5ON-HEqi2OjnCECThGJ73vh7S4qAKspAlxtgBAHFv1Sc_k6Hmdeq_nYXReITCt2FNRy1wWZR-udke9c=@proton.me>
Date: Mon, 07 Apr 2025 23:04:42 +0000
From: Denis Mukhin <dmkhn@...ton.me>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Denis Mukhin <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 v3] x86/early_printk: add MMIO-based UARTs

Hi!

On Monday, April 7th, 2025 at 9:38 AM, Andy Shevchenko <andriy.shevchenko@...el.com> wrote:

> 
> 
> On Mon, Mar 24, 2025 at 05:55:40PM -0700, Denis Mukhin 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.
> > 
> > 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.
> 
> 
> Just for your information: Have you seen this rather old series of mine?
> 
> https://bitbucket.org/andy-shev/linux/commits/branch/topic%2Fx86%2Fboot-earlyprintk

This is a nice cleanup!
Thanks for sharing.

Sorry, I haven't seen the series above, I had to write a patch (which was months ago).
It's just I could not post it on the mailing list until recently.

> 
> --
> With Best Regards,
> Andy Shevchenko
> 

Thanks,
Denis