[maemo-developers] SDL, tearing, X overhead and direct framebuffer gfx

[From: Tapani Pälli]

Hello;

ext Tobias Oberstein wrote:
> Hello tableteers,
>
> I've done some initial experiments hacking my N800/OS2008 and ran into a
> couple of issues:
>
> When using the supplied SDL library for doing timer-based frame
> rendering, there seems to be
>
> - heavy tearing
>   

Tearing unfortunately happens because there is no vsync available for
framebuffer driver to use.

> - significant overhead due to X server
> (rendering pipe is : SDL -> X -> Framebuffer)
>
>   
The overhead here is probably not as big as you expect. SDL uses shared
memory extension to create and update image(s) so all it passes to X is
pointer and then X copies the data to framebuffer driver.

> [Note: yes, I'm aware that doing full screen blits is evil on the N800
> due to limited framebuffer -> video RAM bus bandwidth.]
>
> Q: Is this expected behaviour? What could I do about the tearing? What
> about the Xomap overhead?
>
>   
You cannot help the tearing, you might want to try with different timer
values etc. to make end result look better. If you are planning to
develop a game you could take a look at :

http://maemo.org/community/wiki/gamedevelopment/

You should try to minimize the amount of updated area and redraws.

> ==
>
> I've read a lot of bits on the web 'bout mplayer, vsync, omapfb etc.
> and tried to assemble a minimal example of using direct framebuffer
> access for gfx output.
>
> Q: I can't get the tearing away (only fixed at certain line positions).
> What am I doing wrong?
>
>   
Nothing, you cannot get away from tearing.

> Q: Also, frame rate is sluggish, though CPU load is much lower than
> SDL/X. Ok, my FB example is not threaded like the SDL timer example ..
> is that the reason for framerate even < 15/s?
>
>   

> Q: btw - how can I shutdown Maemo Launcher/Hildon/Matchbox/Xomap?
> Whenever I do one of
>
>   
Quite likely this won't really help you with the performance issues.

> /etc/init.d/maemo-launcher stop
> /etc/init.d/x-server stop
>
> the device will automatically reboot.
>
> ==
>
> I wondered if there would be any plans to make SDL run directly on
> framebuffer .. if not, I'd maybe give it a try.
>
>   
I would be interested in these results too but I wouldn't expect so much
performance boost from there.

> Q: Where can I find the sources to the OS2008 SDL?
>
>   

I wish I knew. Anyway, there is not significant changes in our SDL, you
can safely browse the upstream source at http://libsdl.org/


> Thx and cheers,
>
> tgo
>   


// Tapani Pälli

-- 
Software Engineer
Open Source Software Operations