[PATCH 0/9] drm: Limit DRM_IOCTL_WAIT_VBLANK to vblank interrupts

[PATCH 0/9] drm: Limit DRM_IOCTL_WAIT_VBLANK to vblank interrupts

[Thomas Zimmermann]

DRM's WAIT_VBLANK ioctl synchronizes user-space clients to display
refresh. This is meaningless with vblank timers, which run unrelated
to the hardware's vblank.

Disable the ioctl for simulated vblanks. Set DRM_VBLANK_FLAG_SIMULATED
for CRTCs with simulated vblank events in all such drivers. The vblank
timers of these devices still rate-limit the number of page-flip events
to match the display refresh.

According to maintainers, user-space compositors do not require the ioctl
for rate-limitting display output. Weston and Kwin rely on page-flip
events. Mutter uses and internal timer to limit the number of display
updates per second.

When testing with mutter and weston, the page-flip rate appears correct
with the patch set applied.

This change has been discussed at length on IRC recently.

https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2026-05-08&show_html=true
https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2026-05-12&show_html=true
https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2026-05-13&show_html=true

Thomas Zimmermann (9):
  drm/vblank: Add drmm_vblank_init() to indicate managed cleanup
  drm/vblank: Add DRM_VBLANK_FLAG_SIMULATED
  drm/amdgpu: vkms: Set DRM_VBLANK_FLAG_SIMULATED
  drm/bochs: Set DRM_VBLANK_FLAG_SIMULATED
  drm/cirrus: Set DRM_VBLANK_FLAG_SIMULATED
  drm/hypervdrm: Set DRM_VBLANK_FLAG_SIMULATED
  drm/qxl: Set DRM_VBLANK_FLAG_SIMULATED
  drm/virtgpu: Set DRM_VBLANK_FLAG_SIMULATED
  drm/vkms: Set DRM_VBLANK_FLAG_SIMULATED

 drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c    |  3 ++-
 drivers/gpu/drm/drm_vblank.c                | 19 ++++++++++++-------
 drivers/gpu/drm/drm_vblank_helper.c         |  2 +-
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c |  2 +-
 drivers/gpu/drm/qxl/qxl_display.c           |  2 +-
 drivers/gpu/drm/tiny/bochs.c                |  2 +-
 drivers/gpu/drm/tiny/cirrus-qemu.c          |  2 +-
 drivers/gpu/drm/virtio/virtgpu_display.c    |  2 +-
 drivers/gpu/drm/vkms/vkms_drv.c             |  4 ++--
 include/drm/drm_crtc.h                      |  2 +-
 include/drm/drm_device.h                    |  2 +-
 include/drm/drm_vblank.h                    | 15 ++++++++++++++-
 12 files changed, 38 insertions(+), 19 deletions(-)


base-commit: 121c16f9d8c56ea07263df84ab971cc10870fe88
-- 
2.54.0

[PATCH 1/9] drm/vblank: Add drmm_vblank_init() to indicate managed cleanup

[Thomas Zimmermann]

Rename drm_vblank_init() to drmm_vblank_init(). As the initializer
function sets up managed cleanup, it should use the drmm prefix. Keep
the old name around until all callers have been converted.

Also add a flags argument to the function. The first use of the flags
will be to distinguish between hardware vblank interrupts and simulated
vblank timeouts.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/drm_vblank.c        | 16 +++++++++-------
 drivers/gpu/drm/drm_vblank_helper.c |  2 +-
 include/drm/drm_crtc.h              |  2 +-
 include/drm/drm_device.h            |  2 +-
 include/drm/drm_vblank.h            | 10 +++++++++-
 5 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index f90fb2d13e42..21ca91b4c014 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -117,7 +117,7 @@
  * optionally provide a hardware vertical blanking counter.
  *
  * Drivers must initialize the vertical blanking handling core with a call to
- * drm_vblank_init(). Minimally, a driver needs to implement
+ * drmm_vblank_init(). Minimally, a driver needs to implement
  * &drm_crtc_funcs.enable_vblank and &drm_crtc_funcs.disable_vblank plus call
  * drm_crtc_handle_vblank() in its vblank interrupt handler for working vblank
  * support.
@@ -146,7 +146,7 @@
  * See also DRM vblank helpers for more information.
  *
  * Drivers without support for vertical-blanking interrupts nor timers must
- * not call drm_vblank_init(). For these drivers, atomic helpers will
+ * not call drmm_vblank_init(). For these 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.
@@ -519,7 +519,7 @@ static void vblank_disable_fn(struct timer_list *t)
 	spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
 }
 
-static void drm_vblank_init_release(struct drm_device *dev, void *ptr)
+static void drmm_vblank_init_release(struct drm_device *dev, void *ptr)
 {
 	struct drm_vblank_crtc *vblank = ptr;
 
@@ -534,9 +534,10 @@ static void drm_vblank_init_release(struct drm_device *dev, void *ptr)
 }
 
 /**
- * drm_vblank_init - initialize vblank support
+ * drmm_vblank_init - initialize vblank support
  * @dev: DRM device
  * @num_crtcs: number of CRTCs supported by @dev
+ * @flags: flags for vblank handling
  *
  * This function initializes vblank support for @num_crtcs display pipelines.
  * Cleanup is handled automatically through a cleanup function added with
@@ -545,7 +546,7 @@ static void drm_vblank_init_release(struct drm_device *dev, void *ptr)
  * Returns:
  * Zero on success or a negative error code on failure.
  */
-int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
+int drmm_vblank_init(struct drm_device *dev, unsigned int num_crtcs, unsigned int flags)
 {
 	int ret;
 	unsigned int i;
@@ -564,11 +565,12 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
 
 		vblank->dev = dev;
 		vblank->pipe = i;
+		vblank->flags = flags;
 		init_waitqueue_head(&vblank->queue);
 		timer_setup(&vblank->disable_timer, vblank_disable_fn, 0);
 		seqlock_init(&vblank->seqlock);
 
-		ret = drmm_add_action_or_reset(dev, drm_vblank_init_release,
+		ret = drmm_add_action_or_reset(dev, drmm_vblank_init_release,
 					       vblank);
 		if (ret)
 			return ret;
@@ -580,7 +582,7 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
 
 	return 0;
 }
-EXPORT_SYMBOL(drm_vblank_init);
+EXPORT_SYMBOL(drmm_vblank_init);
 
 /**
  * drm_dev_has_vblank - test if vblanking has been initialized for
diff --git a/drivers/gpu/drm/drm_vblank_helper.c b/drivers/gpu/drm/drm_vblank_helper.c
index d3f8147ecdc1..5b05ab72e133 100644
--- a/drivers/gpu/drm/drm_vblank_helper.c
+++ b/drivers/gpu/drm/drm_vblank_helper.c
@@ -25,7 +25,7 @@
  * for drivers without further requirements. The initializer macro
  * DRM_CRTC_HELPER_VBLANK_FUNCS sets them coveniently.
  *
- * Once the driver enables vblank support with drm_vblank_init(), each
+ * Once the driver enables vblank support with drmm_vblank_init(), each
  * CRTC's vblank timer fires according to the programmed display mode. By
  * default, the vblank timer invokes drm_crtc_handle_vblank(). Drivers with
  * more specific requirements can set their own handler function in
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index c6dbe8b7db9e..f981468d9a00 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -163,7 +163,7 @@ struct drm_crtc_state {
 	 *
 	 * 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() with the number of CRTCs). For CRTCs
+	 * (i.e., call drmm_vblank_init() with the number of CRTCs). For CRTCs
 	 * without initialized vblanking, this field is set to true in
 	 * drm_atomic_helper_check_modeset(), and a fake VBLANK event will be
 	 * send out on each update of the display pipeline by
diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index bc78fb77cc27..381417c6b6f5 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -283,7 +283,7 @@ struct drm_device {
 	 * Array of vblank tracking structures, one per &struct drm_crtc. For
 	 * historical reasons (vblank support predates kernel modesetting) this
 	 * is free-standing and not part of &struct drm_crtc itself. It must be
-	 * initialized explicitly by calling drm_vblank_init().
+	 * initialized explicitly by calling drmm_vblank_init().
 	 */
 	struct drm_vblank_crtc *vblank;
 
diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
index 2fcef9c0f5b1..39a201b83781 100644
--- a/include/drm/drm_vblank.h
+++ b/include/drm/drm_vblank.h
@@ -282,10 +282,12 @@ struct drm_vblank_crtc {
 	 * @vblank_timer: Holds the state of the vblank timer
 	 */
 	struct drm_vblank_crtc_timer vblank_timer;
+
+	unsigned int flags;
 };
 
 struct drm_vblank_crtc *drm_crtc_vblank_crtc(struct drm_crtc *crtc);
-int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs);
+int drmm_vblank_init(struct drm_device *dev, unsigned int num_crtcs, unsigned int flags);
 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,
@@ -321,6 +323,12 @@ int drm_crtc_vblank_start_timer(struct drm_crtc *crtc);
 void drm_crtc_vblank_cancel_timer(struct drm_crtc *crtc);
 void drm_crtc_vblank_get_vblank_timeout(struct drm_crtc *crtc, ktime_t *vblank_time);
 
+/* deprecated; use drmm_vblank_init() instead */
+static inline int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
+{
+	return drmm_vblank_init(dev, num_crtcs, 0);
+}
+
 /*
  * Helpers for struct drm_crtc_funcs
  */
-- 
2.54.0

[PATCH 2/9] drm/vblank: Add DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Add DRM_VBLANK_FLAG_SIMULATED for CRTCs that do not have a hardware
vblank interrupt. Setting the flag tells DRM to not report vblank
capabilities from the WAIT_VBLANK ioctl.

DRM_IOCTL_WAIT_VBLANK queries timestamps from a vblank event or waits
for the next vblank event to occur. DRM clients use this functionality
to synchronize their output with the display's vblank phase. Hence this
is only supported for hardware implementations.

Software implementations are not synchronized to the display and merely
act as a rate limiter for page-flip events. The WAIT_VBLANK ioctl thus
should fail with an error.

Suggested-by: Simona Vetter <simona@ffwll.ch>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/drm_vblank.c | 3 +++
 include/drm/drm_vblank.h     | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 21ca91b4c014..92b699a4e8be 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -1794,6 +1794,9 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
 
 	vblank = drm_vblank_crtc(dev, pipe);
 
+	if (vblank->flags & DRM_VBLANK_FLAG_SIMULATED)
+		return -EOPNOTSUPP;
+
 	/* If the counter is currently enabled and accurate, short-circuit
 	 * queries to return the cached timestamp of the last vblank.
 	 */
diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
index 39a201b83781..03fa7259b6ac 100644
--- a/include/drm/drm_vblank.h
+++ b/include/drm/drm_vblank.h
@@ -37,6 +37,11 @@ struct drm_device;
 struct drm_crtc;
 struct drm_vblank_work;
 
+/**
+ * DRM_VBLANK_FLAG_SIMULATED - vblank uses a software timer
+ */
+#define DRM_VBLANK_FLAG_SIMULATED	BIT(1)
+
 /**
  * struct drm_pending_vblank_event - pending vblank event tracking
  */
-- 
2.54.0

[PATCH 3/9] drm/amdgpu: vkms: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on amdgpu's vkms as simulated, so that the
WAIT_VBLANK ioctl fails with an error. The ioctl should not be
supported because the output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c
index 170adaf7e76a..bc88acc819a6 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c
@@ -413,7 +413,8 @@ static int amdgpu_vkms_sw_init(struct amdgpu_ip_block *ip_block)
 			return r;
 	}
 
-	r = drm_vblank_init(adev_to_drm(adev), adev->mode_info.num_crtc);
+	r = drmm_vblank_init(adev_to_drm(adev), adev->mode_info.num_crtc,
+			     DRM_VBLANK_FLAG_SIMULATED);
 	if (r)
 		return r;
 
-- 
2.54.0

[PATCH 4/9] drm/bochs: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on bochs as simulated, so that the WAIT_VBLANK
ioctl fails with an error. The ioctl should not be supported because
the output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/tiny/bochs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/tiny/bochs.c b/drivers/gpu/drm/tiny/bochs.c
index e2d957e51505..b5955ef39e31 100644
--- a/drivers/gpu/drm/tiny/bochs.c
+++ b/drivers/gpu/drm/tiny/bochs.c
@@ -677,7 +677,7 @@ static int bochs_kms_init(struct bochs_device *bochs)
 	drm_connector_attach_edid_property(connector);
 	drm_connector_attach_encoder(connector, encoder);
 
-	ret = drm_vblank_init(dev, 1);
+	ret = drmm_vblank_init(dev, 1, DRM_VBLANK_FLAG_SIMULATED);
 	if (ret)
 		return ret;
 
-- 
2.54.0

[PATCH 5/9] drm/cirrus: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on cirrus as simulated, so that the WAIT_VBLANK
ioctl fails with an error. The ioctl should not be supported because
the output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/tiny/cirrus-qemu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/tiny/cirrus-qemu.c b/drivers/gpu/drm/tiny/cirrus-qemu.c
index 075221b431d3..01522d1158b2 100644
--- a/drivers/gpu/drm/tiny/cirrus-qemu.c
+++ b/drivers/gpu/drm/tiny/cirrus-qemu.c
@@ -501,7 +501,7 @@ static int cirrus_pipe_init(struct cirrus_device *cirrus)
 	if (ret)
 		return ret;
 
-	ret = drm_vblank_init(dev, 1);
+	ret = drmm_vblank_init(dev, 1, DRM_VBLANK_FLAG_SIMULATED);
 	if (ret)
 		return ret;
 
-- 
2.54.0

[PATCH 6/9] drm/hypervdrm: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on hypervdrm as simulated, so that the WAIT_VBLANK
ioctl fails with an error. The ioctl should not be supported because the
output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
index 1bbb7de5ab49..24bed31c35e7 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
@@ -329,7 +329,7 @@ int hyperv_mode_config_init(struct hyperv_drm_device *hv)
 		return ret;
 	}
 
-	ret = drm_vblank_init(dev, 1);
+	ret = drmm_vblank_init(dev, 1, DRM_VBLANK_FLAG_SIMULATED);
 	if (ret)
 		return ret;
 
-- 
2.54.0

[PATCH 7/9] drm/qxl: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on qxl as simulated, so that the WAIT_VBLANK
ioctl fails with an error. The ioctl should not be supported because
the output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/qxl/qxl_display.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/qxl/qxl_display.c b/drivers/gpu/drm/qxl/qxl_display.c
index a026bd35ef48..b808fdebbd89 100644
--- a/drivers/gpu/drm/qxl/qxl_display.c
+++ b/drivers/gpu/drm/qxl/qxl_display.c
@@ -1300,7 +1300,7 @@ int qxl_modeset_init(struct qxl_device *qdev)
 
 	qxl_display_read_client_monitors_config(qdev);
 
-	ret = drm_vblank_init(&qdev->ddev, qxl_num_crtc);
+	ret = drmm_vblank_init(&qdev->ddev, qxl_num_crtc, DRM_VBLANK_FLAG_SIMULATED);
 	if (ret)
 		return ret;
 
-- 
2.54.0

[PATCH 8/9] drm/virtgpu: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on virtgpu as simulated, so that the WAIT_VBLANK
ioctl fails with an error. The ioctl should not be supported because
the output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/virtio/virtgpu_display.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_display.c b/drivers/gpu/drm/virtio/virtgpu_display.c
index 44ffffec550f..558d8001c54f 100644
--- a/drivers/gpu/drm/virtio/virtgpu_display.c
+++ b/drivers/gpu/drm/virtio/virtgpu_display.c
@@ -381,7 +381,7 @@ int virtio_gpu_modeset_init(struct virtio_gpu_device *vgdev)
 	for (i = 0 ; i < vgdev->num_scanouts; ++i)
 		vgdev_output_init(vgdev, i);
 
-	ret = drm_vblank_init(vgdev->ddev, vgdev->num_scanouts);
+	ret = drmm_vblank_init(vgdev->ddev, vgdev->num_scanouts, DRM_VBLANK_FLAG_SIMULATED);
 	if (ret)
 		return ret;
 
-- 
2.54.0

[PATCH 9/9] drm/vkms: Set DRM_VBLANK_FLAG_SIMULATED

[Thomas Zimmermann]

Mark the vblank event on vkms as simulated, so that the WAIT_VBLANK
ioctl fails with an error. The ioctl should not be supported because
the output is not synchronized to a display refresh.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/vkms/vkms_drv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
index 5a640b531d88..c4cfa1e5ab01 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.c
+++ b/drivers/gpu/drm/vkms/vkms_drv.c
@@ -192,8 +192,8 @@ int vkms_create(struct vkms_config *config)
 		goto out_devres;
 	}
 
-	ret = drm_vblank_init(&vkms_device->drm,
-			      vkms_config_get_num_crtcs(config));
+	ret = drmm_vblank_init(&vkms_device->drm, vkms_config_get_num_crtcs(config),
+			       DRM_VBLANK_FLAG_SIMULATED);
 	if (ret) {
 		DRM_ERROR("Failed to vblank\n");
 		goto out_devres;
-- 
2.54.0

Re: [PATCH 0/9] drm: Limit DRM_IOCTL_WAIT_VBLANK to vblank interrupts

[Michel Dänzer]

On 5/15/26 13:55, Thomas Zimmermann wrote:
> DRM's WAIT_VBLANK ioctl synchronizes user-space clients to display
> refresh. This is meaningless with vblank timers, which run unrelated
> to the hardware's vblank.
> 
> Disable the ioctl for simulated vblanks. Set DRM_VBLANK_FLAG_SIMULATED
> for CRTCs with simulated vblank events in all such drivers. The vblank
> timers of these devices still rate-limit the number of page-flip events
> to match the display refresh.
> 
> According to maintainers, user-space compositors do not require the ioctl
> for rate-limitting display output. Weston and Kwin rely on page-flip
> events. Mutter uses and internal timer to limit the number of display
> updates per second.

Actually mutter fundamentally relies on atomic commit completion events for that, same as Weston & KWin. Mutter uses the WAIT_VBLANK ioctl only for minimizing input → output latency (which can hide issues when completion of atomic commits isn't properly throttled).


(Just a side not on the cover letter, no objections to the patches themselves)


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast

Re: [PATCH 0/9] drm: Limit DRM_IOCTL_WAIT_VBLANK to vblank interrupts

[Michel Dänzer]

[ Adding the wayland-devel list for awareness ]

On 5/15/26 17:12, Michel Dänzer wrote:
> On 5/15/26 13:55, Thomas Zimmermann wrote:
>> DRM's WAIT_VBLANK ioctl synchronizes user-space clients to display
>> refresh. This is meaningless with vblank timers, which run unrelated
>> to the hardware's vblank.
>>
>> Disable the ioctl for simulated vblanks. Set DRM_VBLANK_FLAG_SIMULATED
>> for CRTCs with simulated vblank events in all such drivers. The vblank
>> timers of these devices still rate-limit the number of page-flip events
>> to match the display refresh.
>>
>> According to maintainers, user-space compositors do not require the ioctl
>> for rate-limitting display output. Weston and Kwin rely on page-flip
>> events. Mutter uses and internal timer to limit the number of display
>> updates per second.
> 
> Actually mutter fundamentally relies on atomic commit completion events for that, same as Weston & KWin. Mutter uses the WAIT_VBLANK ioctl only for minimizing input → output latency (which can hide issues when completion of atomic commits isn't properly throttled).
> 
> 
> (Just a side not on the cover letter, no objections to the patches themselves)

After more discussion on IRC, I have some concerns.


The big one first: For drivers with no strict refresh cycle (i.e. an atomic commit can take effect more or less anytime after at least one "refresh cycle" has passed since the last one), does this change really make sense / what's the actual benefit?

In the case of the asahi & nvidia drivers, the problem with exposing this functionality to user space is that if the timestamps aren't accurate, it can result in missing display refresh cycles, which are dictated by hardware. That's why it makes sense to reject it there.

When there's no strict refresh cycle, that issue doesn't apply though.


Any changes made to the WAIT_VBLANK ioctl should also be made to the CRTC_GET_SEQUENCE / CRTC_QUEUE_SEQUENCE ioctls, which are slightly different UAPI for the same functionality.


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast