linux-kernel - Re: drmfb console switching problems/questions

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Date:   Fri, 9 Oct 2020 14:58:59 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     stsp <stsp2@...dex.ru>, linux-fbdev@...r.kernel.org,
        Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: drmfb console switching problems/questions

Hi

Am 09.10.20 um 14:16 schrieb stsp:
> 09.10.2020 14:59, Thomas Zimmermann пишет:
>> Fbdev exposes the video ram (or a shadow buffer of it) to all
>> applications. Only one can draw at the same time. It's a limitation of
>> the design. To fix this, your application has to stop drawing when you
>> switch consoles.
> OK, thanks for info.
> Quite a sad news, if you ask me.
> Is there any ongoing work at solving that?

I don't think so. Both, SDL1 and fbdev, are pretty much dead these days.
The solution to fbdev is DRM, which does not have these problems.

You could your application under X11 to use SDL's X backend. X would do
all the VC handling then. For SDL advice you should ask on the SDL
mailing list.

> 
> Anyway, the fact that fb can
> permanently hang, is, I suppose,
> another problem - the one that
> can't be worked around in an app.
> I suspect it is the result of sdl doing
> ioctl(console_fd, KDSETMODE, KD_GRAPHICS);
> After which, I think, redrawing on a
> VC switch became disabled.
> Would it make sense to improve SAK
> to unfreeze from that? Otherwise its
> too easy to hang the screen permanently,
> and no way to recover.

That 'hung-up' problem you described sounds like the fbdev console does
not correctly redraw itself. But I don't know where the related code is
located.

Best regards
Thomas

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer



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