[Xen-devel] [PATCH v4 01/15] drm: Initialize struct drm_crtc_state.no_vblank from device settings

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

At the end of a commit, atomic helpers can generate a 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
VBLANK generation.

The new interface drm_dev_has_vblank() returns true if vblanking has
been initialized for a device, or false otherwise. This function will
be useful when initializing no_vblank in the CRTC state.

Atomic modesetting helper set the initial value of no_vblank 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 VBLANK events with any
driver that did not initialize vblanking.

v4:
        * replace drm_crtc_has_vblank() with drm_dev_has_vblank()
        * add drm_dev_crtc_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@xxxxxxx>
Acked-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
---
 drivers/gpu/drm/drm_atomic_helper.c | 10 +++++++++-
 drivers/gpu/drm/drm_vblank.c        | 28 ++++++++++++++++++++++++++++
 include/drm/drm_crtc.h              | 27 ++++++++++++++++++++-------
 include/drm/drm_simple_kms_helper.h |  7 +++++--
 include/drm/drm_vblank.h            |  1 +
 5 files changed, 63 insertions(+), 10 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..433dec6230b1 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 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
@@ -501,6 +507,28 @@ int drm_vblank_init(struct drm_device *dev, unsigned int 
num_crtcs)
 }
 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
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 5e9b15a0e8c5..5363e31c9abe 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -174,12 +174,22 @@ struct drm_crtc_state {
         * @no_vblank:
         *
         * Reflects the ability of a CRTC to send VBLANK events. This state
-        * usually depends on the pipeline configuration, and the main usuage
-        * is CRTCs feeding a writeback connector operating in oneshot mode.
-        * 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.
+        * usually depends on the pipeline configuration. If set to true, DRM
+        * atomic helpers will sendout a fake VBLANK event during display
+        * updates.
+        *
+        * One usage is for drivers and/or hardware without support for VBLANK
+        * interrupts. Such drivers typically do not initialize vblanking
+        * (i.e., call drm_vblank_init() wit the number of CRTCs). For CRTCs
+        * without initialized vblanking, the field is initialized to true and
+        * a VBLANK event will be send out on each update of the display
+        * pipeline.
+        *
+        * Another usage is CRTCs feeding a writeback connector operating in
+        * oneshot mode. 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.
         *
         * __drm_atomic_helper_crtc_duplicate_state() will not reset the value
         * from the current state, the CRTC driver is then responsible for
@@ -335,7 +345,10 @@ 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);
diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
index c16c44052b3d..94275e93fd27 100644
--- a/include/drm/drm_vblank.h
+++ b/include/drm/drm_vblank.h
@@ -206,6 +206,7 @@ struct drm_vblank_crtc {
 };
 
 int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs);
+bool drm_dev_has_vblank(const struct drm_device *dev);
 u64 drm_crtc_vblank_count(struct drm_crtc *crtc);
 u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
                                   ktime_t *vblanktime);
-- 
2.24.1


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