[Xen-devel] [PATCH v2 1/4] drm: Document struct drm_crtc_state.no_vblank for faking VBLANK events

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Drivers for CRTC hardware without support for VBLANK interrupts can set
struct drm_crtc_state.no_vblank and let DRM's atomic commit helpers
generate the VBLANK events automatically. Document this in order to make
it official.

Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
---
 drivers/gpu/drm/drm_atomic_helper.c | 4 +++-
 include/drm/drm_crtc.h              | 9 +++++++--
 include/drm/drm_simple_kms_helper.h | 7 +++++--
 3 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c 
b/drivers/gpu/drm/drm_atomic_helper.c
index 4511c2e07bb9..ce30a37971e4 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -2215,7 +2215,9 @@ 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/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 5e9b15a0e8c5..01caf5160596 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -179,7 +179,9 @@ struct drm_crtc_state {
         * In this case the VBLANK event is only generated when a job is queued
         * to the writeback connector, and we want the core to fake VBLANK
         * events when this part of the pipeline hasn't changed but others had
-        * or when the CRTC and connectors are being disabled.
+        * or when the CRTC and connectors are being disabled. In addition to
+        * writeback connectors, this function can also fake VBLANK events for
+        * CRTCs without VBLANK interrupt.
         *
         * __drm_atomic_helper_crtc_duplicate_state() will not reset the value
         * from the current state, the CRTC driver is then responsible for
@@ -335,7 +337,10 @@ struct drm_crtc_state {
         *  - Events for disabled CRTCs are not allowed, and drivers can ignore
         *    that case.
         *
-        * This can be handled by the drm_crtc_send_vblank_event() function,
+        * For very simple hardware without VBLANK interrupt, enabling
+        * &struct drm_crtc_state.no_vblank makes DRM's atomic commit helpers
+        * send the event at an appropriate time. For more complex hardware this
+        * can be handled by the drm_crtc_send_vblank_event() function,
         * which the driver should call on the provided event upon completion of
         * the atomic commit. Note that if the driver supports vblank signalling
         * and timestamping the vblank counters and timestamps must agree with
diff --git a/include/drm/drm_simple_kms_helper.h 
b/include/drm/drm_simple_kms_helper.h
index 15afee9cf049..e253ba7bea9d 100644
--- a/include/drm/drm_simple_kms_helper.h
+++ b/include/drm/drm_simple_kms_helper.h
@@ -100,8 +100,11 @@ struct drm_simple_display_pipe_funcs {
         * This is the function drivers should submit the
         * &drm_pending_vblank_event from. Using either
         * drm_crtc_arm_vblank_event(), when the driver supports vblank
-        * interrupt handling, or drm_crtc_send_vblank_event() directly in case
-        * the hardware lacks vblank support entirely.
+        * interrupt handling, or drm_crtc_send_vblank_event() for more
+        * complex case. In case the hardware lacks vblank support entirely,
+        * drivers can set &struct drm_crtc_state.no_vblank in
+        * &struct drm_simple_display_pipe_funcs.check and let DRM's
+        * atomic helper fake a vblank event.
         */
        void (*update)(struct drm_simple_display_pipe *pipe,
                       struct drm_plane_state *old_plane_state);
-- 
2.24.1


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