linux-kernel - Re: [PATCH next v2 3/4] serial: 8250: Switch to nbcon console

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Message-ID: <84ed5hrnva.fsf@jogness.linutronix.de>
Date: Wed, 18 Sep 2024 17:25:05 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby
 <jirislaby@...nel.org>, Sergey Senozhatsky <senozhatsky@...omium.org>,
 Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner
 <tglx@...utronix.de>, Esben Haabendal <esben@...nix.com>,
 linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org, Andy
 Shevchenko <andriy.shevchenko@...ux.intel.com>, Tony Lindgren
 <tony@...mide.com>, Ilpo Järvinen
 <ilpo.jarvinen@...ux.intel.com>, Uwe
 Kleine-König <u.kleine-koenig@...gutronix.de>, Arnd
 Bergmann
 <arnd@...db.de>, Florian Fainelli <florian.fainelli@...adcom.com>, Serge
 Semin <fancer.lancer@...il.com>, Wolfram Sang
 <wsa+renesas@...g-engineering.com>, Lino Sanfilippo
 <l.sanfilippo@...bus.com>
Subject: Re: [PATCH next v2 3/4] serial: 8250: Switch to nbcon console

On 2024-09-18, Petr Mladek <pmladek@...e.com> wrote:
> It would be fair to mention that it does not longer support fifo in
> the 8250 driver. It basically reverted the commit 8f3631f0f6eb42e5
> ("serial/8250: Use fifo in 8250 console driver").

Agreed.

> It is not usable in write_thread() because it would not allow
> a safe takeover between emitting particular characters.

If write_thread could exit_unsafe()/enter_unsafe() while busy-waiting,
then emergency/panic could still take over at any time. Even if it means
that atomic_write() would need to first wait for the FIFO to drain
(which it will). The important thing is that emergency/panic is able to
take over.

I dropped the optimization to keep things simple for now, but I agree
with Andy that it would be unfortunate. I will take a look at what such
an implementation could look like.

John