linux-kernel - Re: [RFC PATCH v2 1/3] printk: Add function to set console to preferred console's driver

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Date:   Fri, 1 May 2020 10:44:54 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Alper Nebi Yasak <alpernebiyasak@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-serial@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "David S. Miller" <davem@...emloft.net>,
        Feng Tang <feng.tang@...el.com>
Subject: Re: [RFC PATCH v2 1/3] printk: Add function to set console to
 preferred console's driver

On (20/04/30 19:14), Alper Nebi Yasak wrote:
> Currently, add_preferred_console sets a preferred console, but doesn't
> actually change /dev/console to match it. That part is handled within
> register_device, where a newly registered console driver will be set as
> /dev/console if it matches the preferred console.
> 
> However, if the relevant driver is already registered, the only way to
> set it as /dev/console is by un-registering and re-registering it.

Hmm. Preferred console selection is very fragile, there are too many
setups and workarounds that even minor tweaks introduce regressions
oftentimes.

We have, by the way, a pending patchset which changes the same
are - preferred console selection.

git://git.kernel.org/pub/scm/linux/kernel/git/pmladek/printk.git for-5.7-preferred-console

[..]
> An example is the xenfb_make_preferred_console() function:
> 
> 	console_lock();
> 	for_each_console(c) {
> 		if (!strcmp(c->name, "tty") && c->index == 0)
> 			break;
> 	}
> 	console_unlock();
> 	if (c) {
> 		unregister_console(c);
> 		c->flags |= CON_CONSDEV;
> 		c->flags &= ~CON_PRINTBUFFER; /* don't print again */
> 		register_console(c);
> 	}

I didn't know about this code.

> The code above was introduced in commit 9e124fe16ff2 ("xen: Enable
> console tty by default in domU if it's not a dummy"). In short, it's aim
> is to set VT as the preferred console only after a working framebuffer
> is registered and thus VT is not the dummy device.
> 
> This patch introduces an update_console_to_preferred function that
> handles the necessary /dev/console change. With this change, the example
> above can be replaced with:
> 
> 	console_lock();
> 	add_preferred_console("tty", 0, NULL);
> 	update_console_to_preferred();
> 	console_unlock();
> 
> More importantly, these two calls can be moved to vt.c in order to bump
> its priority when a non-dummy backend for it is introduced, solving that
> problem in general.

Let me take a look over the weekend.

	-ss