[PATCH 2/3] drm: Prevent invalid use of vblank_disable_immediate.

[Mario Kleiner]

For a kms driver to support immediate disable of vblank
irq's reliably without introducing off by one errors or
other mayhem for clients, it must not only support a
hardware vblank counter query, but also high precision
vblank timestamping, so vblank count and timestamp can be
instantaneously reinitialzed to valid values. Additionally
the exposed hardware counter must behave as if it is
incrementing at leading edge of vblank to avoid off by
one errors during reinitialization of the counter while
the display happens to be inside or close to vblank.

Check during drm_vblank_init that a driver which claims to
be capable of vblank_disable_immediate at least supports
high precision timestamping and prevent use of instant
disable if that isn't present as a minimum requirement.

Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Michel Dänzer <michel at daenzer.net>
Cc: Thierry Reding <treding at nvidia.com>
Cc: Dave Airlie <airlied at redhat.com>
---
 drivers/gpu/drm/drm_irq.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index af9662e..6efe822 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -336,6 +336,12 @@ int drm_vblank_init(struct drm_device *dev, int num_crtcs)
 	else
 		DRM_INFO("No driver support for vblank timestamp query.\n");
 
+	/* Must have precise timestamping for reliable vblank instant disable */
+	if (dev->vblank_disable_immediate && !dev->driver->get_vblank_timestamp) {
+		dev->vblank_disable_immediate = false;
+		DRM_ERROR("Set vblank_disable_immediate, but not supported.\n");
+	}
+
 	dev->vblank_disable_allowed = false;
 
 	return 0;
-- 
1.9.1