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Tue, 22 Sep 2026 00:26:16 -0700 (PDT) Received: from pathway.suse.cz ([176.114.240.130]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fd8ce2646sm59215695e9.14.2026.09.22.00.26.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Sep 2026 00:26:16 -0700 (PDT) From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Marcos Paulo de Souza , Samuel Thibault , Greg Kroah-Hartman , Jiri Slaby , =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= , Hugo Villeneuve , Fushuai Wang , Kees Cook , Stepan Ionichev , linux-serial@vger.kernel.org, Manuel Lauss , linux-kernel@vger.kernel.org, Petr Mladek Subject: [PATCH 0/2] braille: nbcon: Allow using a serial driver converted to nbcon API as a Braille console Date: Tue, 22 Sep 2026 09:25:56 +0200 Message-ID: <20260922072558.98854-1-pmladek@suse.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi, I have been working on v5 [1] of the patchset cleaning console registration code and I found that the system did not boot when I selected a Braille on the command line, for example: console=brl,ttyS0,115200 console=tty0 Bisection went down to the commit d3539347022a ("serial: 8250: Switch to nbcon console, take 2"). I found that the Braille console driver called braille_co->write() internally where braille_co pointed to the uart 8250 driver. I added the support for the nbcon console API similar way as we did for kdb [2]. Note 1: I believe that we have a bug in the kdb patch because we call con->write() atomic callback after entering the unsafe mode. But we should do so in the default "safe" mode. Note 2: I am not completely sure about the locking. It might be better to use NBCON_PRIO_PANIC in panic(). Also I thought about using con->write_thread(). But I think that we have to use the atomic variant because the Braille console can be called from printk() via the VT code, aka graphical console. Note 3: I do not have any device to read the Braille output so I am not sure if the output is correct. I just checked the output on the serial console and it is a bit suspicious. It looks like it prints every message N-times when N is the length of the string. And it adds one character in each iteration, for example: E>[ E>[ E>[ O *>[ OK A>[ OK A>[ OK A>[ OK ] <>[ OK ] <>[ OK ] R N>[ OK ] Re >[ OK ] Rea J>[ OK ] Reac >[ OK ] Reach A>[ OK ] Reache D>[ OK ] Reached @>[ OK ] Reached @>[ OK ] Reached t But I think that this is not caused by the NBCON console API. [1] https://lore.kernel.org/all/20260604101459.393162-1-pmladek@suse.com/ [2] https://lore.kernel.org/all/20251016-nbcon-kgdboc-v6-0-866aac60a80e@suse.com/ Petr Mladek (2): printk: nbcon: Introduce Braille helpers braille: nbcon: Use nbcon atomic console callbacks .../accessibility/braille/braille_console.c | 38 +++++++++++- drivers/tty/serial/8250/8250_port.c | 5 +- include/linux/console.h | 8 +++ kernel/printk/nbcon.c | 59 +++++++++++++++++++ 4 files changed, 107 insertions(+), 3 deletions(-) -- 2.55.0