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

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Message-ID: <RJ2PbXLplu84G37UUGMyfrvfTrrh5CY5_M6ArUI_eJIl_59Ty7Mqbp4GxJXRd-pjdBuRpqlDy7hak2tcCebae3_lpTqcsrPm-Qpjq4G4wTY=@proton.me>
Date: Tue, 08 Apr 2025 21:03:21 +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

On Tuesday, April 8th, 2025 at 1:30 AM, Andy Shevchenko <andriy.shevchenko@...el.com> wrote:

> 
> 
> On Mon, Apr 07, 2025 at 11:04:42PM +0000, Denis Mukhin wrote:
> 
> > 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.
> 
> 
> No problem. Can you look at
> https://lore.kernel.org/r/20250407172214.792745-1-andriy.shevchenko@linux.intel.com
> ? I forgot to Cc you and that is an important fix.

Done, thanks!

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