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

2 files changed, 37 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 4511c2e07bb9..d7b73cd89b79 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -583,6 +583,7 @@ mode_valid(struct drm_atomic_state *state)
  * &drm_crtc_state.connectors_changed is set when a connector is added or
  * removed from the CRTC.  &drm_crtc_state.active_changed is set when
  * &drm_crtc_state.active changes, which is used for DPMS.
+ * &drm_crtc_state.no_vblank is set from the result of drm_dev_has_vblank().
  * See also: drm_atomic_crtc_needs_modeset()
  *
  * IMPORTANT:
@@ -649,6 +650,11 @@ drm_atomic_helper_check_modeset(struct drm_device *dev,
 
 			return -EINVAL;
 		}
+
+		if (drm_dev_has_vblank(dev))
+			new_crtc_state->no_vblank = false;
+		else
+			new_crtc_state->no_vblank = true;
 	}
 
 	ret = handle_conflicting_encoders(state, false);
@@ -2215,7 +2221,9 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_dependencies);
  * when a job is queued, and any change to the pipeline that does not touch the
  * connector is leading to timeouts when calling
  * drm_atomic_helper_wait_for_vblanks() or
- * drm_atomic_helper_wait_for_flip_done().
+ * drm_atomic_helper_wait_for_flip_done(). In addition to writeback
+ * connectors, this function can also fake VBLANK events for CRTCs without
+ * VBLANK interrupt.
  *
  * This is part of the atomic helper support for nonblocking commits, see
  * drm_atomic_helper_setup_commit() for an overview.
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
  *