Re: Frame swapping

Jenkins, Clive wrote:

quoted

From: Antonino A. Daplas [mailto:adaplas@gmail.com] 
Sent: 08 December 2005 21:18
To: linux-fbdev-devel@lists.sourceforge.net
Cc: Jenkins, Clive
Subject: Re: [Linux-fbdev-devel] Frame swapping

Jenkins, Clive wrote:

quoted

How is frame swapping done, using the mechanisms provided
by FB (in 2.4 and 2.6)?
Can anyone point me to any good code examples in the
existing FB device drivers?

["Frame swapping" or "video page flipping" is switching
to an alternate framebuffer memory area during the
vertical blank period. This is a common technique to
achieve animation or a single clean change of picture.]

Use the pan display ioctl, FBIOPAN_DISPLAY.  Basically, set
var->yres_virtual to be at least 2x var->yres so you can have
2 video buffers, one visible, one offscreen.  Then just do a
ypan ioctl to set which buffer becomes visible and which
becomes offscreen.

Doing it during vblank is a little difficult.  You can poll
the VGA registers (which is slow), or set up an interrupt
handler for the driver you are using that waits for vblank.

See www.directfb.org for code on how this can be done.  Also,
the directfb source have patches to some drivers that implements
the wating for vblank, such as in matroxfb.

Tony

Thanks Tony. I was thinking I would have to implement my own
ioctl to do this, but I agree it can be regarded as a special
case of panning (even though my hardware doesn't fully support
panning).

I would expect panning to be smooth and glitch-free whenever
possible within the limitations of the hardware, so the
switch-over should always occur during vblank. I have seen some
drivers that definitely do this, by deferring all updates until
the vb interrupt. But maybe many other drivers don't bother.

Yes, Because waiting for vblank will slow down console scrolling.

Another way might be to do a GETVAR ioctl, modify "yoffset",
set the FB_ACTIVATE_VBL bit of "activate", and do a SETVAR
ioctl. I am sure this is the intended usage of the ACTIVATE_VBL
bit, even though it may not be well supported by existing drivers.

No, this will be terribly slow.  And most drivers ignore 
ACTIVATE_VBL flag and also resets the yoffset to zero on a
SETVAR ioctl.

I notice in fbmem.c (2.4.20 version), there is no fall-back
code for the PAN_DISPLAY ioctl when the device method is absent.
Perhaps the getvar/modify/setvar above should be coded here.

Possibly.  But don't do this for the reasons mentioned above.
It would be simpler to implement a pan_display hook.

Tony


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