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Peter Anvin" , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] x86/early_printk: add MMIO-based UARTs Message-ID: References: <20250324-earlyprintk-v3-1-aee7421dc469@ford.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250324-earlyprintk-v3-1-aee7421dc469@ford.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo 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