This reverts commit 757055ae8dedf5333af17b3b5b4b70ba9bc9da4e.

The commit caused that ttynull was used as the default console
on several systems[1][2][3]. As a result, the console was
blank even when a better alternative existed.

It happened when there was no console configured
on the command line and ttynull_init() was the first initcall
calling register_console().

Or it happened when /dev/ did not exist when console_on_rootfs()
was called. It was not able to open /dev/console even though
a console driver was registered. It tried to add ttynull console
but it obviously did not help. But ttynull became the preferred
console and was used by /dev/console when it was available later.

The commit tried to fix a historical problem that have been there
for ages. The primary motivation was the commit 3cffa06aeef7ece30f6
("printk/console: Allow to disable console output by using console=""
 or console=null"). It provided a clean solution for a workaround
 that was widely used and worked only by chance.

This revert causes that the console="" or console=null command line
options will again work only by chance. These options will cause that
a particular console will be preferred and the default (tty) ones
will not get enabled. There will be no console registered at
all. As a result there won't be stdin, stdout, and stderr for
the init process. But it worked exactly this way even before.

The proper solution has to fulfill many conditions:

  + Register ttynull only when explicitly required or as
    the ultimate fallback.

  + ttynull should get associated with /dev/console but it must
    not become preferred console when used as a fallback.
    Especially, it must still be possible to replace it
    by a better console later.

Such a change requires clean up of the register_console() code.
Otherwise, it would be even harder to follow. Especially, the use
of has_preferred_console and CON_CONSDEV flag is tricky. The clean
up is risky. The ordering of consoles is not well defined. And
any changes tend to break existing user settings.

Do the revert at the least risky solution for now.

[1] https://lore.kernel.org/linux-kselftest/20201221144302.GR4077@smile.fi.intel.com/
[2] https://lore.kernel.org/lkml/d2a3b3c0-e548-7dd1-730f-59bc5c04e191@synopsys.com/
[3] https://patchwork.ozlabs.org/project/linux-um/patch/20210105120128.10854-1-thomas@m3y3r.de/

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reported-by: Vineet Gupta <vgupta@synopsys.com>
Reported-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Pull printk updates from Petr Mladek:

 - Finally allow parallel writes and reads into/from the lockless
   ringbuffer. But it is not a complete solution. Readers are still
   serialized against each other. And nested writes are still prevented
   by printk_safe per-CPU buffers.

 - Use ttynull as the ultimate fallback for /dev/console.

 - Officially allow disabling console output by using console="" or
   console=null

 - A few code cleanups

* tag 'printk-for-5.11' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  printk: remove logbuf_lock writer-protection of ringbuffer
  printk: inline log_output(),log_store() in vprintk_store()
  printk: remove obsolete dead assignment
  printk/console: Allow to disable console output by using console="" or console=null
  init/console: Use ttynull as a fallback when there is no console
  printk: ringbuffer: Reference text_data_ring directly in callees.

stdin, stdout, and stderr standard I/O stream are created for the init
process. They are not available when there is no console registered
for /dev/console. It might lead to a crash when the init process
tries to use them, see the commit 48021f98130880dd742 ("printk: handle
blank console arguments passed in.").

Normally, ttySX and ttyX consoles are used as a fallback when no consoles
are defined via the command line, device tree, or SPCR. But there
will be no console registered when an invalid console name is configured
or when the configured consoles do not exist on the system.

Users even try to avoid the console intentionally, for example,
by using console="" or console=null. It is used on production
systems where the serial port or terminal are not visible to
users. Pushing messages to these consoles would just unnecessary
slowdown the system.

Make sure that stdin, stdout, stderr, and /dev/console are always
available by a fallback to the existing ttynull driver. It has
been implemented for exactly this purpose but it was used only
when explicitly configured.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20201111135450.11214-2-pmladek@suse.com

Recently in commit 3c4e0dff2095 ("vt: Disable KD_FONT_OP_COPY") we
disabled the KD_FONT_OP_COPY ioctl() option. Delete all the
con_font_copy() callbacks, since we no longer use them.

Mark KD_FONT_OP_COPY as "obsolete" in include/uapi/linux/kd.h, just like
what we have done for PPPIOCDETACH in commit af8d3c7c001a ("ppp: remove
the PPPIOCDETACH ioctl").

Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/c8d28007edf50de4387e1532eb3eb736db716f73.1605169912.git.yepeilin.cs@gmail.com

There are many functions declared in selection.h which only read from
struct vc_data passed as a parameter. Make all those uses const to hint
the compiler a bit.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

VT100ID is unused, but defined twice. Kill it.

VT102ID is used only in respond_ID. Define there a variable with proper
type and use that instead. Then drop both defines of VT102ID too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-9-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

The code currently uses bitfields to store true-false values. Switch all
of that to bools. Apart from the cleanup, it saves 20B of code as many
shifts, ANDs, and ORs became simple movzb's.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-3-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Introduce names (en enum) for 0, 1, and 2 constants. We now have
VCI_HALF_BRIGHT, VCI_NORMAL, and VCI_BOLD instead.

Apart from the cleanup,
1) the enum allows for better type checking, and
2) this saves some code. No more fiddling with bits is needed in
   assembly now. (OTOH, the structure is larger.)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

We need the console patches in here as well for futher work from Andy.

* 'for-5.7-console-exit' of git://git.kernel.org/pub/scm/linux/kernel/git/pmladek/printk:
  console: Introduce ->exit() callback
  console: Don't notify user space when unregister non-listed console
  console: Avoid positive return code from unregister_console()
  console: Drop misleading comment
  console: Use for_each_console() helper in unregister_console()
  console: Drop double check for console_drivers being non-NULL
  console: Don't perform test for CON_BRL flag

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>