[PATCH v2 2/2] fbcon: Defer console takeover for splash screens to first switch

[Mario Limonciello]

On 2/6/2024 08:21, Daniel Vetter wrote:
> On Tue, Feb 06, 2024 at 06:10:51PM +0800, Daniel van Vugt wrote:
>> Until now, deferred console takeover only meant defer until there is
>> output. But that risks stepping on the toes of userspace splash screens,
>> as console messages may appear before the splash screen. So check the
>> command line for the expectation of userspace splash and if present then
>> extend the deferral until after the first switch.
>>
>> V2: Added Kconfig option instead of hard coding "splash".
>>
>> Closes: https://bugs.launchpad.net/bugs/1970069
>> Cc: Mario Limonciello <mario.limonciello at amd.com>
>> Signed-off-by: Daniel van Vugt <daniel.van.vugt at canonical.com>
>> ---
>>   drivers/video/console/Kconfig    | 13 +++++++++++
>>   drivers/video/fbdev/core/fbcon.c | 38 ++++++++++++++++++++++++++++++--
>>   2 files changed, 49 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
>> index bc31db6ef7..a6e371bfb4 100644
>> --- a/drivers/video/console/Kconfig
>> +++ b/drivers/video/console/Kconfig
>> @@ -138,6 +138,19 @@ config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
>>   	  by the firmware in place, rather then replacing the contents with a
>>   	  black screen as soon as fbcon loads.
>>   
>> +config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
>> +	string "Framebuffer Console Deferred Takeover Condition"
>> +	depends on FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
>> +	default "splash"
>> +	help
>> +	  If enabled this defers further the framebuffer console taking over
>> +	  until the first console switch has occurred. And even then only if
>> +	  text has been output, and only if the specified parameter is found
>> +	  on the command line. This ensures fbcon does not interrupt userspace
>> +	  splash screens such as Plymouth which may be yet to start rendering
>> +	  at the time of the first console output. "splash" is the simplest
>> +	  distro-agnostic condition for this that Plymouth checks for.
> 
> Hm this seems a bit strange since a lot of complexity that no one needs,
> also my impression is that it's rather distro specific how you want this
> to work. So why not just a Kconfig option that lets you choose how much
> you want to delay fbcon setup, with the following options:
> 
> - no delay at all
> - dely until first output from the console (which then works for distros
>    which set a log-level to suppress unwanted stuff)
> - delay until first vt-switch. In that case I don't think we also need to
>    delay for first output, since vt switch usually means you'll get first
>    output right away (if it's a vt terminal) or you switch to a different
>    graphical console (which will keep fbcon fully suppressed on the drm
>    side).
> 

IIUC there is an "automatic" VT switch that happens with Ubuntu GRUB + 
Ubuntu kernels.

Why?

Couldn't this also be suppressed by just not doing that?

> I think you could even reuse the existing
> CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER for this, and then just
> compile-time select which kind of notifier to register (well plus the
> check for "splash" on the cmdline for the vt switch one I guess).
> 
> Thoughts?
> 
> Cheers, Sima
> 
> 
>> +
>>   config STI_CONSOLE
>>   	bool "STI text console"
>>   	depends on PARISC && HAS_IOMEM
>> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
>> index 63af6ab034..98155d2256 100644
>> --- a/drivers/video/fbdev/core/fbcon.c
>> +++ b/drivers/video/fbdev/core/fbcon.c
>> @@ -76,6 +76,7 @@
>>   #include <linux/crc32.h> /* For counting font checksums */
>>   #include <linux/uaccess.h>
>>   #include <asm/irq.h>
>> +#include <asm/cmdline.h>
>>   
>>   #include "fbcon.h"
>>   #include "fb_internal.h"
>> @@ -3358,6 +3359,26 @@ static int fbcon_output_notifier(struct notifier_block *nb,
>>   
>>   	return NOTIFY_OK;
>>   }
>> +
>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
>> +static int initial_console;
>> +static struct notifier_block fbcon_switch_nb;
>> +
>> +static int fbcon_switch_notifier(struct notifier_block *nb,
>> +				 unsigned long action, void *data)
>> +{
>> +	struct vc_data *vc = data;
>> +
>> +	WARN_CONSOLE_UNLOCKED();
>> +
>> +	if (vc->vc_num != initial_console) {
>> +		dummycon_unregister_switch_notifier(&fbcon_switch_nb);
>> +		dummycon_register_output_notifier(&fbcon_output_nb);
>> +	}
>> +
>> +	return NOTIFY_OK;
>> +}
>> +#endif
>>   #endif
>>   
>>   static void fbcon_start(void)
>> @@ -3370,7 +3391,16 @@ static void fbcon_start(void)
>>   
>>   	if (deferred_takeover) {
>>   		fbcon_output_nb.notifier_call = fbcon_output_notifier;
>> -		dummycon_register_output_notifier(&fbcon_output_nb);
>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
>> +		if (cmdline_find_option_bool(boot_command_line,
>> +		      CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION)) {
>> +			initial_console = fg_console;
>> +			fbcon_switch_nb.notifier_call = fbcon_switch_notifier;
>> +			dummycon_register_switch_notifier(&fbcon_switch_nb);
>> +		} else
>> +#endif
>> +			dummycon_register_output_notifier(&fbcon_output_nb);
>> +
>>   		return;
>>   	}
>>   #endif
>> @@ -3417,8 +3447,12 @@ void __exit fb_console_exit(void)
>>   {
>>   #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER
>>   	console_lock();
>> -	if (deferred_takeover)
>> +	if (deferred_takeover) {
>>   		dummycon_unregister_output_notifier(&fbcon_output_nb);
>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION
>> +		dummycon_unregister_switch_notifier(&fbcon_switch_nb);
>> +#endif
>> +	}
>>   	console_unlock();
>>   
>>   	cancel_work_sync(&fbcon_deferred_takeover_work);
>> -- 
>> 2.43.0
>>
>