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authorPetr Mladek <pmladek@suse.com>2021-11-22 14:26:48 +0100
committerPetr Mladek <pmladek@suse.com>2021-12-06 14:07:57 +0100
commit4f546939259f8e1130b60553433892774a42ea68 (patch)
tree4271cfd859e83db0cec6e8d37fe3d046344f53fc /kernel/printk
parentf873efe841f813303e8a4af0d4cc48ff1f43bbe2 (diff)
printk/console: Remove need_default_console variable
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: Petr Mladek <pmladek@suse.com> Link: https://lore.kernel.org/r/20211122132649.12737-5-pmladek@suse.com
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c29
1 files changed, 17 insertions, 12 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3f845daa3a4a..6591da285a83 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -280,7 +280,6 @@ static struct console *exclusive_console;
static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
static int preferred_console = -1;
-static bool need_default_console = true;
int console_set_on_cmdline;
EXPORT_SYMBOL(console_set_on_cmdline);
@@ -2919,10 +2918,8 @@ static void try_enable_default_console(struct console *newcon)
newcon->flags |= CON_ENABLED;
- if (newcon->device) {
+ if (newcon->device)
newcon->flags |= CON_CONSDEV;
- need_default_console = false;
- }
}
/*
@@ -2972,16 +2969,24 @@ void register_console(struct console *newcon)
if (console_drivers && console_drivers->flags & CON_BOOT)
bcon = console_drivers;
- if (need_default_console || bcon || !console_drivers)
- need_default_console = preferred_console < 0;
-
/*
- * See if we want to use this console driver. If we
- * didn't select a console we take the first one
- * that registers here.
+ * See if we want to enable this console driver by default.
+ *
+ * Nope when a console is preferred by the command line, device
+ * tree, or SPCR.
+ *
+ * The first real console with tty binding (driver) wins. More
+ * consoles might get enabled before the right one is found.
+ *
+ * Note that a console with tty binding will have CON_CONSDEV
+ * flag set and will be first in the list.
*/
- if (need_default_console)
- try_enable_default_console(newcon);
+ if (preferred_console < 0) {
+ if (!console_drivers || !console_drivers->device ||
+ console_drivers->flags & CON_BOOT) {
+ try_enable_default_console(newcon);
+ }
+ }
/* See if this console matches one we selected on the command line */
err = try_enable_preferred_console(newcon, true);