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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 02.09.25 um 15:27 schrieb Ville Syrjälä:
> 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/

Thanks for this pointer. It has not been fixed yet, right? If vkms is 
affected, this series probably is as well.

Best regards
Thomas

>
>> 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

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)