[PATCH 1/6] drm/vblank: Add vblank timer

[Louis Chauvet]

Le 05/06/2025 à 17:24, Thomas Zimmermann a écrit :
> 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").
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>

Tested-by: Louis Chauvet <louis.chauvet at bootlin.com>
Reviewed-by: Louis Chauvet <louis.chauvet at bootlin.com>

> ---
>   drivers/gpu/drm/Makefile           |   3 +-
>   drivers/gpu/drm/drm_vblank_timer.c | 100 +++++++++++++++++++++++++++++
>   include/drm/drm_vblank_timer.h     |  26 ++++++++
>   3 files changed, 128 insertions(+), 1 deletion(-)
>   create mode 100644 drivers/gpu/drm/drm_vblank_timer.c
>   create mode 100644 include/drm/drm_vblank_timer.h
> 
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index b5d5561bbe5f..6722e2d1aa7e 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -146,7 +146,8 @@ 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_timer.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_timer.c b/drivers/gpu/drm/drm_vblank_timer.c
> new file mode 100644
> index 000000000000..be46d3135c8e
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_vblank_timer.c
> @@ -0,0 +1,100 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +
> +#include <linux/hrtimer.h>
> +
> +#include <drm/drm_crtc.h>
> +#include <drm/drm_managed.h>
> +#include <drm/drm_print.h>
> +#include <drm/drm_vblank.h>
> +#include <drm/drm_vblank_timer.h>
> +
> +static enum hrtimer_restart drm_vblank_timer_function(struct hrtimer *timer)
> +{
> +	struct drm_vblank_timer *vtimer = container_of(timer, struct drm_vblank_timer, timer);
> +	struct drm_crtc *crtc = vtimer->crtc;
> +	struct drm_device *dev = crtc->dev;
> +	u64 ret_overrun;
> +	bool succ;
> +
> +	ret_overrun = hrtimer_forward_now(&vtimer->timer, vtimer->period_ns);
> +	if (ret_overrun != 1)
> +		drm_warn(dev, "vblank timer overrun\n");
> +
> +	if (vtimer->crtc_handle_vblank)
> +		succ = vtimer->crtc_handle_vblank(crtc);
> +	else
> +		succ = drm_crtc_handle_vblank(crtc);
> +	if (!succ)
> +		return HRTIMER_NORESTART;
> +
> +	return HRTIMER_RESTART;
> +}
> +
> +static void drmm_vblank_timer_release(struct drm_device *dev, void *res)
> +{
> +	struct drm_vblank_timer *vtimer = res;
> +
> +	hrtimer_cancel(&vtimer->timer);
> +}
> +
> +int drmm_vblank_timer_init(struct drm_vblank_timer *vtimer, struct drm_crtc *crtc,
> +			   bool (*crtc_handle_vblank)(struct drm_crtc *crtc))
> +{
> +	struct hrtimer *timer = &vtimer->timer;
> +
> +	vtimer->crtc = crtc;
> +	vtimer->crtc_handle_vblank = crtc_handle_vblank;
> +
> +	hrtimer_setup(timer, drm_vblank_timer_function, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> +
> +	return drmm_add_action_or_reset(crtc->dev, drmm_vblank_timer_release, vtimer);
> +}
> +EXPORT_SYMBOL(drmm_vblank_timer_init);
> +
> +void drm_vblank_timer_start(struct drm_vblank_timer *vtimer)
> +{
> +	struct drm_crtc *crtc = vtimer->crtc;
> +	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> +
> +	drm_calc_timestamping_constants(crtc, &crtc->mode);
> +
> +	vtimer->period_ns = ktime_set(0, vblank->framedur_ns);
> +	hrtimer_start(&vtimer->timer, vtimer->period_ns, HRTIMER_MODE_REL);
> +}
> +EXPORT_SYMBOL(drm_vblank_timer_start);
> +
> +void drm_vblank_timer_cancel(struct drm_vblank_timer *vtimer)
> +{
> +	hrtimer_cancel(&vtimer->timer);
> +}
> +EXPORT_SYMBOL(drm_vblank_timer_cancel);
> +
> +bool drm_vblank_timer_get_vblank_timestamp(struct drm_vblank_timer *vtimer,
> +					   int *max_error, ktime_t *vblank_time,
> +					   bool in_vblank_irq)
> +{
> +	struct drm_crtc *crtc = vtimer->crtc;
> +	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> +
> +	if (!READ_ONCE(vblank->enabled)) {
> +		*vblank_time = ktime_get();
> +		return true;
> +	}
> +
> +	*vblank_time = READ_ONCE(vtimer->timer.node.expires);
> +
> +	if (WARN_ON(*vblank_time == vblank->time))
> +		return true;
> +
> +	/*
> +	 * 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
> +	 * timestampe by one frame.
> +	 */
> +	*vblank_time -= vtimer->period_ns;
> +
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_vblank_timer_get_vblank_timestamp);
> diff --git a/include/drm/drm_vblank_timer.h b/include/drm/drm_vblank_timer.h
> new file mode 100644
> index 000000000000..0b827ff1f59c
> --- /dev/null
> +++ b/include/drm/drm_vblank_timer.h
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +
> +#ifndef _DRM_VBLANK_TIMER_H_
> +#define _DRM_VBLANK_TIMER_H_
> +
> +#include <linux/hrtimer_types.h>
> +#include <linux/types.h>
> +
> +struct drm_crtc;
> +
> +struct drm_vblank_timer {
> +	struct drm_crtc *crtc;
> +	bool (*crtc_handle_vblank)(struct drm_crtc *crtc);
> +	ktime_t period_ns;
> +	struct hrtimer timer;
> +};
> +
> +int drmm_vblank_timer_init(struct drm_vblank_timer *vtimer, struct drm_crtc *crtc,
> +			   bool (*handle_vblank)(struct drm_crtc *crtc));
> +void drm_vblank_timer_start(struct drm_vblank_timer *vtimer);
> +void drm_vblank_timer_cancel(struct drm_vblank_timer *vtimer);
> +bool drm_vblank_timer_get_vblank_timestamp(struct drm_vblank_timer *vtimer,
> +					   int *max_error, ktime_t *vblank_time,
> +					   bool in_vblank_irq);
> +
> +#endif

-- 
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com