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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
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