[Intel-gfx] [PATCH 4/6] drm/i915: Disable FBC across page-flipping

[Jesse Barnes]

On Thu,  7 Jul 2011 12:48:09 +0100
Chris Wilson <chris at chris-wilson.co.uk> wrote:

> Page-flipping updates the scanout address, nukes the FBC compressed
> image and so forces an FBC update so that the displayed image remains
> consistent. However, page-flipping does not update the FBC registers
> themselves, which remain pointing to both the old address and the old
> CPU fence. Future updates to the new front-buffer (scanout) are then
> undetected!
> 
> This first approach to demonstrate the issue and highlight the fix,
> simply disables FBC upon page-flip (a recompression will be forced on
> every flip so FBC becomes immaterial) and then re-enables FBC in the
> page-flip finish work function, so that the FBC registers are now
> pointing to the new framebuffer and front-buffer rendering works once
> more.
> 
> Ideally, we want to only re-enable FBC after page-flipping is complete,
> as otherwise we are just wasting cycles (along with an undesirable
> wait-for-vblank which will halve the frame-rate of vsync'ed games) and
> power (with needless recompression) whilst the page-flipping application
> is still running.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=33487
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---

Yeah just disabling it is probably better than trying to let the hw
figure out what to recompress since it likely won't be worthwhile.

Reviewed-by: Jesse Barnes <jbarnes at virtuousgeek.org>

-- 
Jesse Barnes, Intel Open Source Technology Center