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

[Javier Martinez Canillas]

Thomas Zimmermann <tzimmermann at 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 at suse.de>
> Reported-by: Andrei Borzenkov <arvidjaar at gmail.com>
> Closes: https://bugzilla.suse.com/show_bug.cgi?id=1242191
> Tested-by: Andrei Borzenkov <arvidjaar at gmail.com>
> Fixes: 83d83bebf401 ("console/fbcon: Add support for deferred console takeover")
> Cc: Hans de Goede <hdegoede at redhat.com>
> Cc: linux-fbdev at vger.kernel.org
> Cc: dri-devel at lists.freedesktop.org
> Cc: <stable at 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.

> +	return true;
> +}

Acked-by: Javier Martinez Canillas <javierm at redhat.com>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat