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

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Message-ID: <Z_P_nWrl4JQJVy2c@black.fi.intel.com>
Date: Mon, 7 Apr 2025 19:38:53 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Denis Mukhin <dmukhin@...d.com>
Cc: 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

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

-- 
With Best Regards,
Andy Shevchenko