Swap limit

Mario Kleiner mario.kleiner at tuebingen.mpg.de
Fri Dec 10 06:40:38 PST 2010

On 12/08/2010 05:15 PM, Pauli Nieminen wrote:
> On 08/12/10 16:55 +0100, ext Alex Deucher wrote:
>> One other thing that might be worth adding to DRI2 is a way for the
>> driver to access the swap interval.  If we could, then the driver
>> could dynamically disable things like vline waits for buffer blits or
>> do non-vsynced pageflipping more easily if the swap interval was 0.
> Actually I tough DRI2 was already telling SwapInteval with
> *swap_target = pPriv->last_swap_target + pPriv->swap_interval;
> If swap_target doesn't advance then driver would know to flip as fast as
> possible.
> But then I noticed a bug.
> First one is that we always blit with swapinterval is zero when it should be
> possible to flip (even tear free).

Let it tear :). I think swap_interval == 0 should cause immediate swaps 
with tearing. All OpenGL implementations i know (Windows, MacOS classic, 
MacOS/X, Linux with proprietary drivers, old SGI's) interpret a 
swapinterval of zero as "swap as soon as rendering is finished, 
immediately, don't sync to scanout cycle" and i think that makes the 
most sense. It's good for benchmarking how fast your system can go if 
not throttled by the monitor (useful), for crazy gamers that trade 
visual quality for fps and for special applications that need to 
sometimes control swap timing themselves (like my toolkit).

> Code from DRI2ScheduleSwap:
> /* Old DDX or no swap interval, just blit */
> if (!ds->ScheduleSwap || !pPriv->swap_interval) {

Not a "bug", just a half-done way of achieving the expected 
swap_interval zero ;-) [ok, maybe that is a bug].

It schedules an immediate copy-swap via blitting. Unfortunately the ddx 
doesn't know about the swap_interval, so it still synchronizes the 
execution of the blit to vsync via vline waits. That's tear-free, but it 
depends on the location and size of the drawable and the current 
position of the scanout if this will cause an immediate swap (if scanout 
is outside the drawables area) or a vsync'ed swap. It's a bit undefined 
behaviour for non-fullscreen drawables and it effectively enforces a 
minimum swap interval of 1 for fullscreen drawables, which is not what 
we want.

Both the intel ddx and (soon) the ati ddx have a xorg.conf parameter 
"SwapBuffersWait" to disable vsync completely, then you'd get copy-swaps 
as fast as possible for swap_interval zero, but for a non-zero 
swap_interval you'd have some chance of tearing, so this is alos just a 

If we wanted this for page-flipping we'd need a shortcut similar to this 
one which would bypass the vblank scheduling and call the ddx pageflip 
routine directly, plus an extension to the kernel's pageflip ioctl() to 
allow non-vsynced flips.

And we need a new interface to tell the ddx that a swap should be 


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