[PATCH] drm/vkms: add missing drm_crtc_vblank_put to the get/put pair on flush

[Melissa Wen]

This patch adds a missing drm_crtc_vblank_put op to the pair
drm_crtc_vblank_get/put (inc/decrement counter to guarantee vblanks).

It clears the execution of the following kms_cursor_crc subtests:
1. pipe-A-cursor-[size,alpha-opaque, NxN-(on-screen, off-screen, sliding,
   random, fast-moving])] - successful when running individually.
2. pipe-A-cursor-dpms passes again
3. pipe-A-cursor-suspend also passes

The issue was initially tracked in the sequential execution of IGT
kms_cursor_crc subtest: when running the test sequence or one of its
subtests twice, the odd execs complete and the pairs get stuck in an
endless wait. In the IGT code, calling a wait_for_vblank before the start
of CRC capture prevented the busy-wait. But the problem persisted in the
pipe-A-cursor-dpms and -suspend subtests.

Checking the history, the pipe-A-cursor-dpms subtest was successful when,
in vkms_atomic_commit_tail, instead of using the flip_done op, it used
wait_for_vblanks. Another way to prevent blocking was wait_one_vblank when
enabling crtc. However, in both cases, pipe-A-cursor-suspend persisted
blocking in the 2nd start of CRC capture, which may indicate that
something got stuck in the step of CRC setup. Indeed, wait_one_vblank in
the crc setup was able to sync things and free all kms_cursor_crc
subtests.

Tracing and comparing a clean run with a blocked one:
- in a clean one, vkms_crtc_atomic_flush enables vblanks;
- when blocked, only in next op, vkms_crtc_atomic_enable, the vblanks
started. Moreover, a series of vkms_vblank_simulate flow out until
disabling vblanks.
Also watching the steps of vkms_crtc_atomic_flush, when the very first
drm_crtc_vblank_get returned an error, the subtest crashed. On the other
hand, when vblank_get succeeded, the subtest completed. Finally, checking
the flush steps: it increases counter to hold a vblank reference (get),
but there isn't a op to decreased it and release vblanks (put).

Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
Cc: Haneen Mohammed <hamohammed.sa at gmail.com>
Signed-off-by: Melissa Wen <melissa.srw at gmail.com>
---
 drivers/gpu/drm/vkms/vkms_crtc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
index ac85e17428f8..a99d6b4a92dd 100644
--- a/drivers/gpu/drm/vkms/vkms_crtc.c
+++ b/drivers/gpu/drm/vkms/vkms_crtc.c
@@ -246,6 +246,7 @@ static void vkms_crtc_atomic_flush(struct drm_crtc *crtc,
 
 		spin_unlock(&crtc->dev->event_lock);
 
+		drm_crtc_vblank_put(crtc);
 		crtc->state->event = NULL;
 	}
 
-- 
2.27.0