[PATCH i915 v7 0/2] PRIME Synchronization

[Alex Goins]

Hello all,

For a while now, I've been working to fix tearing with PRIME. This is the
same as the sixth version of the DRM component for PRIME synchronization,
but with my mail headers fixed so that the patches can be checked in.

In sixth version I modified the wait in prepare_plane_fb to properly handle
interrupts by returning -ERESTARTSYS, and warn on other error codes before
continuing.

Due to the inability to properly handle interrupts in mmio_flip_work_func,
I changed it back to an uninterruptible wait, matching the semantics used
by __i915_wait_request(). Like prepare_plane_fb, I also changed it to warn
on error codes.

Repeat of overview below:

v1 was a more complicated patch set that added an additional fenced
interface to page flipping. To avoid adding additional interfaces on top of
a legacy path, v2 scrapped those patches and changed i915 page flipping
paths to wait on fences attached to DMA-BUF-backed fbs. Subsequent versions
involve incremental changes outlined in the patch descriptions.

I have two patches, one that implements fencing for i915's legacy mmio_flip
path, and one for atomic modesetting for futureproofing. Currently the
mmio_flip path is the one ultimately used by the X patches, due to the lack
of asynchronous atomic modesetting support in i915.

With my synchronization patches to X, it is possible to export two shared
buffers per crtc instead of just one. The sink driver uses the legacy
drmModePageFlip() to flip between the buffers, as the rest of the driver
has yet to be ported to atomics. In the pageflip/vblank event handler, the
sink driver requests a present from the source using the new X ABI function
pScreen->PresentTrackedFlippingPixmap(). If the call returns successfully,
it uses drmModePageFlip() to flip to the updated buffer, otherwise it waits
until the next vblank and tries again.

When the source driver presents on a given buffer, it first attaches a
fence.  The source driver is responsible for either using software
signaling or hardware semaphore-backed fences to ensure the fence is
signaled when the present is finished. If the sink's DRM driver implements
fencing in the flipping path, it will guarantee that that flip won't occur
until the present has finished.

This means that DRM drivers that don't implement fencing in their flipping
paths won't be able to guarantee 100% tear-free PRIME with my X patches.
However, the good news is that even without fencing, tearing is rare.
Generally presenting finishes before the next vblank, so there is no need
to wait on the fence. The X patches are a drastic improvement with or
without fencing, but the fencing is nonetheless important to guarantee
tear-free under all conditions.

To give some greater context, I've uploaded my branches for DRM and the X
server to Github. I'll move forward with upstreaming the X changes if and
when these DRM patches go in.

DRM Tree:    https://github.com/GoinsWithTheWind/drm-prime-sync
X Tree:      https://github.com/GoinsWithTheWind/xserver-prime-sync

(branch agoins-prime-v6)

Thanks, Alex @ NVIDIA Linux Driver Team

Alex Goins (2):
  i915: wait for fence in mmio_flip_work_func
  i915: wait for fence in prepare_plane_fb

 drivers/gpu/drm/i915/intel_display.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

-- 
1.9.1