Re: [PATCH] dummycon: Trigger redraw when switching consoles with deferred takeover

[Thomas Zimmermann <tzimmermann@suse.de>]

Am 19.05.25 um 11:04 schrieb Javier Martinez Canillas:
> Thomas Zimmermann <tzimmermann@suse.de> writes:
>
> Hello Thomas,
>
>> Signal vt subsystem to redraw console when switching to dummycon
>> with deferred takeover enabled. Makes the console switch to fbcon
>> and displays the available output.
>>
>> With deferred takeover enabled, dummycon acts as the placeholder
>> until the first output to the console happens. At that point, fbcon
>> takes over. If the output happens while dummycon is not active, it
>> cannot inform fbcon. This is the case if the vt subsystem runs in
>> graphics mode.
>>
>> A typical graphical boot starts plymouth, a display manager and a
>> compositor; all while leaving out dummycon. Switching to a text-mode
>> console leaves the console with dummycon even if a getty terminal
>> has been started.
>>
>> Returning true from dummycon's con_switch helper signals the vt
>> subsystem to redraw the screen. If there's output available dummycon's
>> con_putc{s} helpers trigger deferred takeover of fbcon, which sets a
>> display mode and displays the output. If no output is available,
>> dummycon remains active.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> Reported-by: Andrei Borzenkov <arvidjaar@gmail.com>
>> Closes: https://bugzilla.suse.com/show_bug.cgi?id=1242191
>> Tested-by: Andrei Borzenkov <arvidjaar@gmail.com>
>> Fixes: 83d83bebf401 ("console/fbcon: Add support for deferred console takeover")
>> Cc: Hans de Goede <hdegoede@redhat.com>
>> Cc: linux-fbdev@vger.kernel.org
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: <stable@vger.kernel.org> # v4.19+
>> ---
>>   drivers/video/console/dummycon.c | 15 ++++++++++-----
>>   1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/video/console/dummycon.c b/drivers/video/console/dummycon.c
>> index 139049368fdc..afb8e4d2fc34 100644
>> --- a/drivers/video/console/dummycon.c
>> +++ b/drivers/video/console/dummycon.c
>> @@ -85,6 +85,12 @@ static bool dummycon_blank(struct vc_data *vc, enum vesa_blank_mode blank,
>>   	/* Redraw, so that we get putc(s) for output done while blanked */
>>   	return true;
>>   }
>> +
>> +static bool dummycon_switch(struct vc_data *vc)
>> +{
>> +	/* Redraw, so that we get putc(s) for output done while switched away */
> Maybe this comment could be a little bit more verbose about why this is needed
> for the framebuffer console deferred takeover case? It doesn't have to be as
> elaborated as how you have it in the commit message, but more information would
> be nice IMO.

I copied that from dummycon_blank(), as I assumed that the relevant 
people know why. But yeah, I can elaborate a bit more. Best regards Thomas
>
>> +	return true;
>> +}
> Acked-by: Javier Martinez Canillas <javierm@redhat.com>
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)