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

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Message-ID: <ZurG8YMmBmVVxttj@pathway.suse.cz>
Date: Wed, 18 Sep 2024 14:26:25 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
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 Fri 2024-09-13 16:11:37, John Ogness wrote:
> Implement the necessary callbacks to switch the 8250 console driver
> to perform as an nbcon console.
> 
> Add implementations for the nbcon console callbacks (write_atomic,
> write_thread, device_lock, device_unlock) and add CON_NBCON to the
> initial flags.
> 
> All register access in the callbacks are within unsafe sections.
> The write_thread() callback allows safe handover/takeover per byte.
> The write_atomic() callback allows safe handover/takeover per
> printk record and adds a preceding newline if it took over mid-line.
> 
> For the write_atomic() case, a new irq_work is used to defer modem
> control since it may be a context that does not allow waking up
> tasks.

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").

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

It might still be used in write_atomic() but it is probably not
worth it. This callback is used "only" in emergency and panic
situations.

> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Otherwise, it looks good to me. And it even works fine.
With an updated commit message:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr