[PATCH] drm/vkms: Avoid extra discount in the timestamp value

[Rodrigo Siqueira]

After the commit def35e7c5926 ("drm/vkms: Bugfix extra vblank frame")
some of the crc tests (IGT) start to fail in the vkms with the following
error:

 [drm:drm_crtc_add_crc_entry [drm]] *ERROR* Overflow of CRC buffer,
    userspace reads too slow.
 [drm] failed to queue vkms_crc_work_handle
 ...

The commit def35e7c5926 ("drm/vkms: Bugfix extra vblank frame") added an
operation for decrement the timestamp value returned by
`get_vblank_timestamp()` which solved the problems related to kms_flip
tests. However, each call to `get_vblank_timestamp()` reduces the
timestamp even when it is not required. Such decrement generates a
negative number in the following calculation within
`drm_update_vblank_count()`:

 u64 diff_ns = ktime_to_ns(ktime_sub(t_vblank, vblank->time));

Next, the DIV_ROUND_CLOSEST_ULL macro is invoked using the diff_ns
value, which generates an undefined result. Therefore, the returned
frame is incorrect, which assign a huge number to the variable
frame_end. In this case, causing the loop (`vkms_crc_work_handle()`)
below to take a long time:

 frame_end = drm_crtc_accurate_vblank_count(crtc);
 while (frame_start <= frame_end)
   drm_crtc_add_crc_entry(crtc, true, frame_start++, &crc32);

This commit fixes this issue by adding a check that validates whether
the current vblank timestamp is still the same, thus avoiding excessive
timestamp decrement of the  timestamp value.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
Signed-off-by: Shayenne Moura <shayenneluzmoura at gmail.com>
---
 drivers/gpu/drm/vkms/vkms_crtc.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
index 7508815fac11..3ce60e66673e 100644
--- a/drivers/gpu/drm/vkms/vkms_crtc.c
+++ b/drivers/gpu/drm/vkms/vkms_crtc.c
@@ -74,9 +74,13 @@ bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
 {
 	struct vkms_device *vkmsdev = drm_device_to_vkms_device(dev);
 	struct vkms_output *output = &vkmsdev->output;
+	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
 
 	*vblank_time = output->vblank_hrtimer.node.expires;
 
+	if (*vblank_time == vblank->time)
+		return true;
+
 	if (!in_vblank_irq)
 		*vblank_time -= output->period_ns;
 
-- 
2.21.0
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