[meta-freescale] Sync to V-blank

Carlos Rafael Giani dv at pseudoterminal.org
Thu Apr 3 03:32:06 PDT 2014

This X11 vsync problem has been bugging several people, and to date, 
there is no proper solution (other than to use framebuffer directly 
instead, or switch to Wayland).

Eric Nelson mentioned details about this, as well as a possible 
workaround. I appended his email here:


On 03/18/2014 02:39 PM, Carlos Rafael Giani wrote:
> Hello,
> in the past, there has always been a problem with vsync, OpenGL ES
> output, and X11. Very noticeable tearing affects all such applications.
> So far, neither Vivante nor Freescale have ever commented on this. Is
> there anything known? Should the newest drivers fix this?

You have odd timing. I was just looking into this today for a

In our kernels and the Freescale kernels. there seems to be an
issue with the default allocation of the frame-buffer, such that
only space for  a single buffer is present.


Without a larger allocation, I don't think the frame buffer
driver has any way of swapping cleanly at vertical sync.

The 3.10.17-beta kernel seems to do the same thing:

You can see this at run-time by looking in

     # cat /sys/class/graphics/fb0/mode
     # cat /sys/class/graphics/fb0/virtual_size

You can also alter things by using sysfs:
     # echo 1280,1600 > /sys/class/graphics/fb0/virtual_size

After changing the size, the FBIO_PAN ioctl does the right
thing. i.e.:

     variable_info.yoffset = x;
     err = ioctl(fdfb,FBIOPAN_DISPLAY,&variable_info);

And you can see the same thing using /sys/class/graphics/fb0/pan.

It's not clear to me who should be doing this though.

In an X environment, I would expect this to be controlled
through xorg.conf somehow, but my attempts to get fbdev
and shadowfb proved fruitless.

I haven't had a chance to look into the Vivante X driver
and I'm not sure where the OpenGL stack this should be
performed, but since the timing really has to be coordinated
by the frame-buffer, there should be a call somewhere.

In my immediate customer case, using the frame buffer calls
directly is sufficient.



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