[PATCH 1/2] drm/vkms: Bugfix extra vblank frame

[Shayenne Moura]

kms_flip tests are breaking on vkms when simulate vblank because vblank
event sequence count returns one extra frame after arm vblank event to
make a page flip.

When vblank interrupt happens, userspace processes the vblank event and
issues the next page flip command. Kernel calls queue_work to call
commit_planes and arm the new page flip. The next vblank picks up the
newly armed vblank event and vblank interrupt happens again.

The arm and vblank event are asynchronous, then, on the next vblank, we
receive x+2 from `get_vblank_timestamp`, instead x+1, although timestamp
and vblank seqno matches.

Function `get_vblank_timestamp` is reached by 2 ways:

  - from `drm_mode_page_flip_ioctl`: driver is doing one atomic operation
    to synchronize planes in the same output. There is no vblank simulation,
    the `drm_crtc_arm_vblank_event` function adds 1 on vblank count, and the
    variable in_vblank_irq is false
  - from `vkms_vblank_simulate`: since the driver is doing a vblank simulation,
    the variable in_vblank_irq is true.

Fix this problem subtracting one vblank period from vblank_time when
`get_vblank_timestamp` is called from trace `drm_mode_page_flip_ioctl`,
i.e., is not a real vblank interrupt, and getting the timestamp and vblank
seqno when it is a real vblank interrupt.

The reason for all this is that get_vblank_timestamp always supplies the
timestamp for the next vblank event. The hrtimer is the vblank simulator,
and it needs the correct previous value to present the next vblank. Since
this is how hw timestamp registers work and what the vblank core expects.

Signed-off-by: Shayenne Moura <shayenneluzmoura at gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

---
 drivers/gpu/drm/vkms/vkms_crtc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
index d44bfc392491..23146ff2a25b 100644
--- a/drivers/gpu/drm/vkms/vkms_crtc.c
+++ b/drivers/gpu/drm/vkms/vkms_crtc.c
@@ -87,6 +87,9 @@ bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
 
 	*vblank_time = output->vblank_hrtimer.node.expires;
 
+	if (!in_vblank_irq)
+		*vblank_time -= output->period_ns;
+
 	return true;
 }
 
-- 
2.17.1