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    • Petr Mladek's avatar
      printk/console: Clean up boot console handling in register_console() · 5e8ba485
      Petr Mladek authored 
      The variable @bcon has two meanings. It is used several times for iterating
the list of registered consoles. In the meantime, it holds the information
whether a boot console is first in @console_drivers list.

The information about the 1st console driver used to be important for
the decision whether to install the new console by default or not.
It allowed to re-evaluate the variable @need_default_console when
a real console with tty binding has been unregistered in the meantime.

The decision about the default console is not longer affected by @bcon
variable. The current code checks whether the first driver is real
and has tty binding directly.

The information about the first console is still used for two more
decisions:

  1. It prevents duplicate output on non-boot consoles with
     CON_CONSDEV flag set.

  2. Early/boot consoles are unregistered when a real console with
     CON_CONSDEV is registered and @keep_bootcon is not set.

The behavior in the real life is far from obvious. @bcon is set according
to the first console @console_drivers list. But the first position in
the list is special:

  1. Consoles with CON_CONSDEV flag are put at the beginning of
     the list. It is either the preferred console or any console
     with tty binding registered by default.

  2. Another console might become the first in the list when
     the first console in the list is unregistered. It might
     happen either explicitly or automatically when boot
     consoles are unregistered.

There is one more important rule:

  + Boot consoles can't be registered when any real console
    is already registered.

It is a puzzle. The main complication is the dependency on the first
position is the list and the complicated rules around it.

Let's try to make it easier:

1. Add variable @bootcon_enabled and set it by iterating all registered
   consoles. The variable has obvious meaning and more predictable
   behavior. Any speed optimization and other tricks are not worth it.

2. Use a generic name for the variable that is used to iterate
   the list on registered console drivers.

Behavior change:

No, maybe surprisingly, there is _no_ behavior change!

Let's provide the proof by contradiction. Both operations, duplicate
output prevention and boot consoles removal, are done only when
the newly added console has CON_CONSDEV flag set. The behavior
would change when the new @bootcon_enabled has different value
than the original @bcon.

By other words, the behavior would change when the following conditions
are true:

   + a console with CON_CONSDEV flag is added
   + a real (non-boot) console is the first in the list
   + a boot console is later in the list

Now, a real console might be first in the list only when:

   + It was the first registered console. In this case, there can't be
     any boot console because any later ones were rejected.

   + It was put at the first position because it had CON_CONSDEV flag
     set. It was either the preferred console or it was a console with
     tty binding registered by default. We are interested only in
     a real consoles here. And real console with tty binding fulfills
     conditions of the default console.

     Now, there is always only one console that is either preferred
     or fulfills conditions of the default console. It can't be already
     in the list and being registered at the same time.

As a result, the above three conditions could newer be "true" at
the same time. Therefore the behavior can't change.

Final dilemma:

OK, the new code has the same behavior. But is the change in the right
direction? What if the handling of @console_drivers is updated in
the future?

OK, let's look at it from another angle:

1. The ordering of @console_drivers list is important only in
   console_device() function. The first console driver with tty
   binding gets associated with /dev/console.

2. CON_CONSDEV flag is shown in /proc/consoles. And it should be set
   for the driver that is returned by console_device().

3. A boot console is removed and the duplicated output is prevented
   when the real console with CON_CONSDEV flag is registered.

Now, in the ideal world:

+ The driver associated with /dev/console should be either a console
  preferred via the command line, device tree, or SPCR. Or it should
  be the first real console with tty binding registered by default.

+ The code should match the related boot and real console drivers.
  It should unregister only the obsolete boot driver. And the duplicated
  output should be prevented only on the related real driver.

It is clear that it is not guaranteed by the current code. Instead,
the current code looks like a maze of heuristics that try to achieve
the above.

It is result of adding several features over last few decades. For example,
a possibility to register more consoles, unregister consoles, boot
consoles, consoles without tty binding, device tree, SPCR, braille
consoles.

Anyway, there is no reason why the decision, about removing boot consoles
and preventing duplicated output, should depend on the first console
in the list. The current code does the decisions primary by CON_CONSDEV
flag that is used for the preferred console. It looks like a
good compromise. And the change seems to be in the right direction.
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20211122132649.12737-6-pmladek@suse.com
      5e8ba485
    • Petr Mladek's avatar
      printk/console: Remove need_default_console variable · 4f546939
      Petr Mladek authored 
      The variable @need_default_console is used to decide whether a newly
registered console should get enabled by default.

The logic is complicated. It can be modified in a register_console()
call. But it is always re-evaluated in the next call by the following
condition:

	if (need_default_console || bcon || !console_drivers)
		need_default_console = preferred_console < 0;

In short, the value is updated when either of the condition is valid:

  + the value is still, or again, "true"
  + boot/early console is still the first in @console_driver list
  + @console_driver list is empty

The value is updated according to @preferred_console. In particular,
it is set to "false" when a @preferred_console was set by
__add_preferred_console(). This happens when a non-braille console
was added via the command line, device tree, or SPCR.

It far from clear what this all means together. Let's look at
@need_default_console from another angle:

1. The value is "true" by default. It means that it is always set
   according to @preferred_console during the first register_console()
   call.

   By other words, the first register_console() call will register
   the console by default only when none non-braille console was defined
   via the command line, device tree, or SPCR.

2. The value will always stay "false" when @preferred_console is set.

   By other words, try_enable_default_console() will never get called
   when a non-braille console is explicitly required.

4. The value might be set to "false" in try_enable_default_console()
   when a console with tty binding (driver) gets enabled.

   In this case CON_CONSDEV is set as well. It causes that the console
   will be inserted as first into the list @console_driver. It might
   be either real or boot/early console.

5. The value will be set _back_ to "true" in the next register_console()
   call when:

      + The console added by the previous register_console() had been
	a boot/early one.

      + The last console has been unregistered in the meantime and
	a boot/early console became first in @console_drivers list
	again. Or the list became empty.

   By other words, the value will stay "false" only when the last
   registered console was real, had tty binding, and was not removed
   in the mean time.

The main logic looks clear:

  + Consoles are enabled by default only when no one is preferred
    via the command line, device tree, or SPCR.

  + By default, any console is enabled until a real console
    with tty binding gets registered.

The behavior when the real console with tty binding is later removed
is a bit unclear:

  + By default, any new console is registered again only when there
    is no console or the first console in the list is a boot one.

The question is why the code is suddenly happy when a real console
without tty binding is the first in the list. It looks like an overlook
and bug.

Conclusion:

The state of @preferred_console and the first console in @console_driver
list should be enough to decide whether we need to enable the given console
by default.

The rules are simple. New consoles are _not_ enabled by default
when either of the following conditions is true:

  + @preferred_console is set. It means that a non-braille console
    is explicitly configured via the command line, device tree, or SPCR.

  + A real console with tty binding is registered. Such a console will
    have CON_CONSDEV flag set and will always be the first in
    @console_drivers list.

Note:

The new code does not use @bcon variable. The meaning of the variable
is far from clear. The direct check of the first console in the list
makes it more clear that only real console fulfills requirements
of the default console.

Behavior change:

As already discussed above. There was one situation where the original
code worked a strange way. Let's have:

	+ console A: real console without tty binding
	+ console B: real console with tty binding

and do:

	register_console(A);	/* 1st step */
	register_console(B);	/* 2nd step */
	unregister_console(B);	/* 3rd step */
	register_console(B);	/* 4th step */

The original code will not register the console B in the 4th step.
@need_default_console is set to "false" in 2nd step. The real console
with tty binding (driver) is then removed in the 3rd step.
But @need_default_console will stay "false" in the 4th step because
there is no boot/early console and @registered_consoles list is not
empty.

The new code will register the console B in the 4th step because
it checks whether the first console has tty binding (->driver)

This behavior change should acceptable:

  1. The scenario requires manual intervention (console removal).
     The system should boot with the same consoles as before.

  2. Console B is registered again probably because the user wants
     to use it. The most likely scenario is that the related
     module is reloaded.

  3. It makes the behavior more consistent and predictable.
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20211122132649.12737-5-pmladek@suse.com
      4f546939
    • Petr Mladek's avatar
      printk/console: Remove unnecessary need_default_console manipulation · f873efe8
      Petr Mladek authored 
      There is no need to clear @need_default_console when a console
preferred by the command line, device tree, or SPCR, gets enabled.

The code is called only when some non-braille console matched a console
in @console_cmdline array. It means that a non-braille console was added
in __add_preferred_console() and the variable preferred_console is set
to a number >= 0. As a result, @need_default_console is always set to
"false" in the magic condition:

	if (need_default_console || bcon || !console_drivers)
		need_default_console = preferred_console < 0;

This is one small step in removing the above magic condition
that is hard to follow.

The patch removes one superfluous assignment and should not change
the functionality.
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20211122132649.12737-4-pmladek@suse.com
      f873efe8
    • Petr Mladek's avatar
      printk/console: Rename has_preferred_console to need_default_console · a6953370
      Petr Mladek authored 
      The logic around the variable @has_preferred_console made my head
spin many times. Part of the problem is the ambiguous name.

There is the variable @preferred_console. It points to the last
non-braille console in @console_cmdline array. This array contains
consoles preferred via the command line, device tree, or SPCR.

Then there is the variable @has_preferred_console. It is set to
"true" when @preferred_console is enabled or when a console with
tty binding gets enabled by default.

It might get reset back by the magic condition:

	if (!has_preferred_console || bcon || !console_drivers)
		has_preferred_console = preferred_console >= 0;

It is a puzzle. Dumb explanation is that it gets re-evaluated
when:

	+ it was not set before (see above when it gets set)
	+ there is still an early console enabled (bcon)
	+ there is no console enabled (!console_drivers)

This is still a puzzle.

It gets more clear when we see where the value is checked. The only
meaning of the variable is to decide whether we should try to enable
the new console by default.

Rename the variable according to the single situation where
the value is checked.

The rename requires an inverted logic. Otherwise, it is a simple
search & replace. It does not change the functionality.
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Reviewed-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Link: https://lore.kernel.org/r/20211122132649.12737-3-pmladek@suse.com
      a6953370
    • Petr Mladek's avatar
      printk/console: Split out code that enables default console · ed758b30
      Petr Mladek authored 
      Put the code enabling a console by default into a separate function
called try_enable_default_console().

Rename try_enable_new_console() to try_enable_preferred_console() to
make the purpose of the different variants more clear.

It is a code refactoring without any functional change.
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Reviewed-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Link: https://lore.kernel.org/r/20211122132649.12737-2-pmladek@suse.com
      ed758b30
    • Christophe JAILLET's avatar
      vsprintf: Use non-atomic bitmap API when applicable · 52e68cd6
      Christophe JAILLET authored 
      The 'set' bitmap is local to this function. No concurrent access to it is
possible.
So prefer the non-atomic '__[set|clear]_bit()' function to save a few
cycles.
Signed-off-by: default avatarChristophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/1abf81a5e509d372393bd22041eed4ebc07ef9f7.1638023178.git.christophe.jaillet@wanadoo.fr
      52e68cd6
  9. 18 Nov, 2021 2 commits
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  13. 16 Nov, 2021 4 commits
  15. 15 Nov, 2021 8 commits
  17. 14 Nov, 2021 12 commits
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