Re: [PATCH v2 1/4] drm/vblank: Add vblank timer

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Sep 01, 2025 at 01:06:58PM +0200, Thomas Zimmermann wrote:
> The vblank timer simulates a vblank interrupt for hardware without
> support. Rate-limits the display update frequency.
> 
> DRM drivers for hardware without vblank support apply display updates
> ASAP. A vblank event informs DRM clients of the completed update.
> 
> Userspace compositors immediately schedule the next update, which
> creates significant load on virtualization outputs. Display updates
> are usually fast on virtualization outputs, as their framebuffers are
> in regular system memory and there's no hardware vblank interrupt to
> throttle the update rate.
> 
> The vblank timer is a HR timer that signals the vblank in software.
> It limits the update frequency of a DRM driver similar to a hardware
> vblank interrupt. The timer is not synchronized to the actual vblank
> interval of the display.
> 
> The code has been adopted from vkms, which added the funtionality
> in commit 3a0709928b17 ("drm/vkms: Add vblank events simulated by
> hrtimers").

Does this suffer from the same deadlocks as well?
https://lore.kernel.org/all/20250510094757.4174662-1-zengheng4@huawei.com/

> 
> The new implementation is part of the existing vblank support,
> which sets up the timer automatically. Drivers only have to start
> and cancel the vblank timer as part of enabling and disabling the
> CRTC. The new vblank helper library provides callbacks for struct
> drm_crtc_funcs.
> 
> The standard way for handling vblank is to call drm_crtc_handle_vblank().
> Drivers that require additional processing, such as vkms, can init
> handle_vblank_timeout in struct drm_crtc_helper_funcs to refer to
> their timeout handler.
> 
> v2:
> - implement vblank timer entirely in vblank helpers
> - downgrade overrun warning to debug
> - fix docs
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Tested-by: Louis Chauvet <louis.chauvet@bootlin.com>
> Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
> ---
>  Documentation/gpu/drm-kms-helpers.rst    |  12 +++
>  drivers/gpu/drm/Makefile                 |   3 +-
>  drivers/gpu/drm/drm_vblank.c             | 122 ++++++++++++++++++++++-
>  drivers/gpu/drm/drm_vblank_helper.c      |  96 ++++++++++++++++++
>  include/drm/drm_modeset_helper_vtables.h |  12 +++
>  include/drm/drm_vblank.h                 |  28 ++++++
>  include/drm/drm_vblank_helper.h          |  33 ++++++
>  7 files changed, 303 insertions(+), 3 deletions(-)
>  create mode 100644 drivers/gpu/drm/drm_vblank_helper.c
>  create mode 100644 include/drm/drm_vblank_helper.h
> 
> diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
> index 5139705089f2..781129f78b06 100644
> --- a/Documentation/gpu/drm-kms-helpers.rst
> +++ b/Documentation/gpu/drm-kms-helpers.rst
> @@ -92,6 +92,18 @@ GEM Atomic Helper Reference
>  .. kernel-doc:: drivers/gpu/drm/drm_gem_atomic_helper.c
>     :export:
>  
> +VBLANK Helper Reference
> +-----------------------
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_vblank_helper.c
> +   :doc: overview
> +
> +.. kernel-doc:: include/drm/drm_vblank_helper.h
> +   :internal:
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_vblank_helper.c
> +   :export:
> +
>  Simple KMS Helper Reference
>  ===========================
>  
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index 4dafbdc8f86a..5ba4ffdb8055 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -150,7 +150,8 @@ drm_kms_helper-y := \
>  	drm_plane_helper.o \
>  	drm_probe_helper.o \
>  	drm_self_refresh_helper.o \
> -	drm_simple_kms_helper.o
> +	drm_simple_kms_helper.o \
> +	drm_vblank_helper.o
>  drm_kms_helper-$(CONFIG_DRM_PANEL_BRIDGE) += bridge/panel.o
>  drm_kms_helper-$(CONFIG_DRM_FBDEV_EMULATION) += drm_fb_helper.o
>  obj-$(CONFIG_DRM_KMS_HELPER) += drm_kms_helper.o
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 46f59883183d..2a4ee41e2fcf 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -136,8 +136,17 @@
>   * 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
> + * Drivers for hardware without support for vertical-blanking interrupts can
> + * use DRM vblank timers to send vblank events at the rate of the current
> + * display mode's refresh. While not synchronized to the hardware's
> + * vertical-blanking regions, the timer helps DRM clients and compositors to
> + * adapt their update cycle to the display output. Drivers should set up
> + * vblanking as usual, but call drm_crtc_vblank_start_timer() and
> + * drm_crtc_vblank_cancel_timer() as part of their atomic mode setting.
> + * 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
>   * 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.
> @@ -2162,3 +2171,112 @@ int drm_crtc_queue_sequence_ioctl(struct drm_device *dev, void *data,
>  	return ret;
>  }
>  
> +/*
> + * VBLANK timer
> + */
> +
> +static enum hrtimer_restart drm_vblank_timer_function(struct hrtimer *timer)
> +{
> +	struct drm_vblank_crtc_timer *vtimer =
> +		container_of(timer, struct drm_vblank_crtc_timer, timer);
> +	struct drm_crtc *crtc = vtimer->crtc;
> +	const struct drm_crtc_helper_funcs *crtc_funcs = crtc->helper_private;
> +	struct drm_device *dev = crtc->dev;
> +	u64 ret_overrun;
> +	bool succ;
> +
> +	ret_overrun = hrtimer_forward_now(&vtimer->timer, vtimer->interval);
> +	if (ret_overrun != 1)
> +		drm_dbg_vbl(dev, "vblank timer overrun\n");
> +
> +	if (crtc_funcs->handle_vblank_timeout)
> +		succ = crtc_funcs->handle_vblank_timeout(crtc);
> +	else
> +		succ = drm_crtc_handle_vblank(crtc);
> +	if (!succ)
> +		return HRTIMER_NORESTART;
> +
> +	return HRTIMER_RESTART;
> +}
> +
> +/**
> + * drm_crtc_vblank_start_timer - Starts the vblank timer on the given CRTC
> + * @crtc: the CRTC
> + *
> + * Drivers should call this function from their CRTC's enable_vblank
> + * function to start a vblank timer. The timer will fire after the duration
> + * of a full frame. drm_crtc_vblank_cancel_timer() disables a running timer.
> + *
> + * Returns:
> + * 0 on success, or a negative errno code otherwise.
> + */
> +int drm_crtc_vblank_start_timer(struct drm_crtc *crtc)
> +{
> +	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> +	struct drm_vblank_crtc_timer *vtimer = &vblank->vblank_timer;
> +
> +	if (!vtimer->crtc) {
> +		vtimer->crtc = crtc;
> +		hrtimer_setup(&vtimer->timer, drm_vblank_timer_function,
> +			      CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> +	}
> +
> +	drm_calc_timestamping_constants(crtc, &crtc->mode);
> +
> +	vtimer->interval = ktime_set(0, vblank->framedur_ns);
> +	hrtimer_start(&vtimer->timer, vtimer->interval, HRTIMER_MODE_REL);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_crtc_vblank_start_timer);
> +
> +/**
> + * drm_crtc_vblank_start_timer - Cancels the given CRTC's vblank timer
> + * @crtc: the CRTC
> + *
> + * Drivers should call this function from their CRTC's disable_vblank
> + * function to stop a vblank timer.
> + */
> +void drm_crtc_vblank_cancel_timer(struct drm_crtc *crtc)
> +{
> +	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> +	struct drm_vblank_crtc_timer *vtimer = &vblank->vblank_timer;
> +
> +	hrtimer_cancel(&vtimer->timer);
> +}
> +EXPORT_SYMBOL(drm_crtc_vblank_cancel_timer);
> +
> +/**
> + * drm_crtc_vblank_get_vblank_timeout - Returns the vblank timeout
> + * @crtc: The CRTC
> + * @vblank_time: Returns the next vblank timestamp
> + *
> + * The helper drm_crtc_vblank_get_vblank_timeout() returns the next vblank
> + * timestamp of the CRTC's vblank timer according to the timer's expiry
> + * time.
> + */
> +void drm_crtc_vblank_get_vblank_timeout(struct drm_crtc *crtc, ktime_t *vblank_time)
> +{
> +	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> +	struct drm_vblank_crtc_timer *vtimer = &vblank->vblank_timer;
> +
> +	if (!READ_ONCE(vblank->enabled)) {
> +		*vblank_time = ktime_get();
> +		return;
> +	}
> +
> +	*vblank_time = READ_ONCE(vtimer->timer.node.expires);
> +
> +	if (drm_WARN_ON(crtc->dev, !ktime_compare(*vblank_time, vblank->time)))
> +		return; /* Already expired */
> +
> +	/*
> +	 * To prevent races we roll the hrtimer forward before we do any
> +	 * interrupt processing - this is how real hw works (the interrupt
> +	 * is only generated after all the vblank registers are updated)
> +	 * and what the vblank core expects. Therefore we need to always
> +	 * correct the timestamp by one frame.
> +	 */
> +	*vblank_time = ktime_sub(*vblank_time, vtimer->interval);
> +}
> +EXPORT_SYMBOL(drm_crtc_vblank_get_vblank_timeout);
> diff --git a/drivers/gpu/drm/drm_vblank_helper.c b/drivers/gpu/drm/drm_vblank_helper.c
> new file mode 100644
> index 000000000000..f94d1e706191
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_vblank_helper.c
> @@ -0,0 +1,96 @@
> +// SPDX-License-Identifier: MIT
> +
> +#include <drm/drm_crtc.h>
> +#include <drm/drm_managed.h>
> +#include <drm/drm_modeset_helper_vtables.h>
> +#include <drm/drm_print.h>
> +#include <drm/drm_vblank.h>
> +#include <drm/drm_vblank_helper.h>
> +
> +/**
> + * DOC: overview
> + *
> + * The vblank helper library provides functions for supporting vertical
> + * blanking in DRM drivers.
> + *
> + * For vblank timers, several callback implementations are available.
> + * Drivers enable support for vblank timers by setting the vblank callbacks
> + * in struct &drm_crtc_funcs to the helpers provided by this library. The
> + * initializer macro DRM_CRTC_VBLANK_TIMER_FUNCS does this conveniently.
> + *
> + * Once the driver enables vblank support with drm_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
> + * struct &drm_crtc_helper_funcs.handle_vblank_timeout.
> + */
> +
> +/*
> + * VBLANK timer
> + */
> +
> +/**
> + * drm_crtc_vblank_helper_enable_vblank_timer - Implements struct &drm_crtc_funcs.enable_vblank
> + * @crtc: The CRTC
> + *
> + * The helper drm_crtc_vblank_helper_enable_vblank_timer() implements
> + * enable_vblank of struct drm_crtc_helper_funcs for CRTCs that require
> + * a VBLANK timer. It sets up the timer on the first invocation. The
> + * started timer expires after the current frame duration. See struct
> + * &drm_vblank_crtc.framedur_ns.
> + *
> + * See also struct &drm_crtc_helper_funcs.enable_vblank.
> + *
> + * Returns:
> + * 0 on success, or a negative errno code otherwise.
> + */
> +int drm_crtc_vblank_helper_enable_vblank_timer(struct drm_crtc *crtc)
> +{
> +	return drm_crtc_vblank_start_timer(crtc);
> +}
> +EXPORT_SYMBOL(drm_crtc_vblank_helper_enable_vblank_timer);
> +
> +/**
> + * drm_crtc_vblank_helper_disable_vblank_timer - Implements struct &drm_crtc_funcs.disable_vblank
> + * @crtc: The CRTC
> + *
> + * The helper drm_crtc_vblank_helper_disable_vblank_timer() implements
> + * disable_vblank of struct drm_crtc_funcs for CRTCs that require a
> + * VBLANK timer.
> + *
> + * See also struct &drm_crtc_helper_funcs.disable_vblank.
> + */
> +void drm_crtc_vblank_helper_disable_vblank_timer(struct drm_crtc *crtc)
> +{
> +	drm_crtc_vblank_cancel_timer(crtc);
> +}
> +EXPORT_SYMBOL(drm_crtc_vblank_helper_disable_vblank_timer);
> +
> +/**
> + * drm_crtc_vblank_helper_get_vblank_timestamp_from_timer -
> + *	Implements struct &drm_crtc_funcs.get_vblank_timestamp
> + * @crtc: The CRTC
> + * @max_error: Maximum acceptable error
> + * @vblank_time: Returns the next vblank timestamp
> + * @in_vblank_irq: True is called from drm_crtc_handle_vblank()
> + *
> + * The helper drm_crtc_helper_get_vblank_timestamp_from_timer() implements
> + * get_vblank_timestamp of struct drm_crtc_funcs for CRTCs that require a
> + * VBLANK timer. It returns the timestamp according to the timer's expiry
> + * time.
> + *
> + * See also struct &drm_crtc_funcs.get_vblank_timestamp.
> + *
> + * Returns:
> + * True on success, or false otherwise.
> + */
> +bool drm_crtc_vblank_helper_get_vblank_timestamp_from_timer(struct drm_crtc *crtc,
> +							    int *max_error,
> +							    ktime_t *vblank_time,
> +							    bool in_vblank_irq)
> +{
> +	drm_crtc_vblank_get_vblank_timeout(crtc, vblank_time);
> +
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_crtc_vblank_helper_get_vblank_timestamp_from_timer);
> diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> index ce7c7aeac887..fe32854b7ffe 100644
> --- a/include/drm/drm_modeset_helper_vtables.h
> +++ b/include/drm/drm_modeset_helper_vtables.h
> @@ -490,6 +490,18 @@ struct drm_crtc_helper_funcs {
>  				     bool in_vblank_irq, int *vpos, int *hpos,
>  				     ktime_t *stime, ktime_t *etime,
>  				     const struct drm_display_mode *mode);
> +
> +	/**
> +	 * @handle_vblank_timeout: Handles timeouts of the vblank timer.
> +	 *
> +	 * Called by CRTC's the vblank timer on each timeout. Semantics is
> +	 * equivalient to drm_crtc_handle_vblank(). Implementations should
> +	 * invoke drm_crtc_handle_vblank() as part of processing the timeout.
> +	 *
> +	 * This callback is optional. If unset, the vblank timer invokes
> +	 * drm_crtc_handle_vblank() directly.
> +	 */
> +	bool (*handle_vblank_timeout)(struct drm_crtc *crtc);
>  };
>  
>  /**
> diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
> index 151ab1e85b1b..f020415abd20 100644
> --- a/include/drm/drm_vblank.h
> +++ b/include/drm/drm_vblank.h
> @@ -25,6 +25,7 @@
>  #define _DRM_VBLANK_H_
>  
>  #include <linux/seqlock.h>
> +#include <linux/hrtimer.h>
>  #include <linux/idr.h>
>  #include <linux/poll.h>
>  #include <linux/kthread.h>
> @@ -103,6 +104,24 @@ struct drm_vblank_crtc_config {
>  	bool disable_immediate;
>  };
>  
> +/**
> + * struct drm_vblank_crtc_timer - vblank timer for a CRTC
> + */
> +struct drm_vblank_crtc_timer {
> +	/**
> +	 * @timer: The vblank's high-resolution timer
> +	 */
> +	struct hrtimer timer;
> +	/**
> +	 * @interval: Duration between two vblanks
> +	 */
> +	ktime_t interval;
> +	/**
> +	 * @crtc: The timer's CRTC
> +	 */
> +	struct drm_crtc *crtc;
> +};
> +
>  /**
>   * struct drm_vblank_crtc - vblank tracking for a CRTC
>   *
> @@ -254,6 +273,11 @@ struct drm_vblank_crtc {
>  	 * cancelled.
>  	 */
>  	wait_queue_head_t work_wait_queue;
> +
> +	/**
> +	 * @vblank_timer: Holds the state of the vblank timer
> +	 */
> +	struct drm_vblank_crtc_timer vblank_timer;
>  };
>  
>  struct drm_vblank_crtc *drm_crtc_vblank_crtc(struct drm_crtc *crtc);
> @@ -290,6 +314,10 @@ wait_queue_head_t *drm_crtc_vblank_waitqueue(struct drm_crtc *crtc);
>  void drm_crtc_set_max_vblank_count(struct drm_crtc *crtc,
>  				   u32 max_vblank_count);
>  
> +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);
> +
>  /*
>   * Helpers for struct drm_crtc_funcs
>   */
> diff --git a/include/drm/drm_vblank_helper.h b/include/drm/drm_vblank_helper.h
> new file mode 100644
> index 000000000000..74a971d0cfba
> --- /dev/null
> +++ b/include/drm/drm_vblank_helper.h
> @@ -0,0 +1,33 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +
> +#ifndef _DRM_VBLANK_HELPER_H_
> +#define _DRM_VBLANK_HELPER_H_
> +
> +#include <linux/hrtimer_types.h>
> +#include <linux/types.h>
> +
> +struct drm_crtc;
> +
> +/*
> + * VBLANK timer
> + */
> +
> +int drm_crtc_vblank_helper_enable_vblank_timer(struct drm_crtc *crtc);
> +void drm_crtc_vblank_helper_disable_vblank_timer(struct drm_crtc *crtc);
> +bool drm_crtc_vblank_helper_get_vblank_timestamp_from_timer(struct drm_crtc *crtc,
> +							    int *max_error,
> +							    ktime_t *vblank_time,
> +							    bool in_vblank_irq);
> +
> +/**
> + * DRM_CRTC_VBLANK_TIMER_FUNCS - Default implementation for VBLANK timers
> + *
> + * This macro initializes struct &drm_crtc_funcs to default helpers for
> + * VBLANK timers.
> + */
> +#define DRM_CRTC_VBLANK_TIMER_FUNCS \
> +	.enable_vblank = drm_crtc_vblank_helper_enable_vblank_timer, \
> +	.disable_vblank = drm_crtc_vblank_helper_disable_vblank_timer, \
> +	.get_vblank_timestamp = drm_crtc_vblank_helper_get_vblank_timestamp_from_timer
> +
> +#endif
> -- 
> 2.50.1

-- 
Ville Syrjälä
Intel