drm: Initialize struct drm_crtc_state.no_vblank from device settings

At the end of a commit, atomic helpers can generate a fake VBLANK event
automatically. Originally implemented for writeback connectors, the
functionality can be used by any driver and/or hardware without proper
VBLANK interrupt.

The patch updates the documentation to make this behaviour official:
settings struct drm_crtc_state.no_vblank to true enables automatic
generation of fake VBLANK events.

The new interface drm_dev_has_vblank() returns true if vblanking has
been initialized for a device, or false otherwise. Atomic helpers use
this function when initializing no_vblank in the CRTC state in
drm_atomic_helper_check_modeset(). If vblanking has been initialized
for a device, no_blank is disabled. Otherwise it's enabled. Hence,
atomic helpers will automatically send out fake VBLANK events with any
driver that did not initialize vblanking.

v5:
	* more precise documentation and commit message
v4:
	* replace drm_crtc_has_vblank() with drm_dev_has_vblank()
	* add drm_dev_has_vblank() in this patch
	* move driver changes into separate patches
v3:
	* squash all related changes patches into this patch

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200129120531.6891-2-tzimmermann@suse.de

1 files changed, 28 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 1659b13b178c..27a99c486f02 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -69,6 +69,12 @@
  * &drm_driver.max_vblank_count. In that case the vblank core only disables the
  * vblanks after a timer has expired, which can be configured through the
  * ``vblankoffdelay`` module parameter.
+ *
+ * Drivers for hardware without support for vertical-blanking interrupts
+ * must not call drm_vblank_init(). For such drivers, atomic helpers will
+ * automatically generate fake vblank events as part of the display update.
+ * This functionality also can be controlled by the driver by enabling and
+ * disabling struct drm_crtc_state.no_vblank.
  */
 
 /* Retry timestamp calculation up to 3 times to satisfy
@@ -502,6 +508,28 @@ err:
 EXPORT_SYMBOL(drm_vblank_init);
 
 /**
+ * drm_dev_has_vblank - test if vblanking has been initialized for
+ *                      a device
+ * @dev: the device
+ *
+ * Drivers may call this function to test if vblank support is
+ * initialized for a device. For most hardware this means that vblanking
+ * can also be enabled.
+ *
+ * Atomic helpers use this function to initialize
+ * &drm_crtc_state.no_vblank. See also drm_atomic_helper_check_modeset().
+ *
+ * Returns:
+ * True if vblanking has been initialized for the given device, false
+ * otherwise.
+ */
+bool drm_dev_has_vblank(const struct drm_device *dev)
+{
+	return dev->num_crtcs != 0;
+}
+EXPORT_SYMBOL(drm_dev_has_vblank);
+
+/**
  * drm_crtc_vblank_waitqueue - get vblank waitqueue for the CRTC
  * @crtc: which CRTC's vblank waitqueue to retrieve
  *