linux-kernel - Re: [PATCH v4 10/14] drm/vblank: Add helper to get next vblank time

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Date:   Wed, 22 Feb 2023 05:37:42 -0500
From:   Luben Tuikov <luben.tuikov@....com>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     Rob Clark <robdclark@...omium.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        open list <linux-kernel@...r.kernel.org>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        freedreno@...ts.freedesktop.org
Subject: Re: [PATCH v4 10/14] drm/vblank: Add helper to get next vblank time

On 2023-02-18 16:15, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.org>
> 
> Will be used in the next commit to set a deadline on fences that an
> atomic update is waiting on.
> 
> Signed-off-by: Rob Clark <robdclark@...omium.org>
> ---
>  drivers/gpu/drm/drm_vblank.c | 32 ++++++++++++++++++++++++++++++++
>  include/drm/drm_vblank.h     |  1 +
>  2 files changed, 33 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 2ff31717a3de..caf25ebb34c5 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -980,6 +980,38 @@ u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
>  }
>  EXPORT_SYMBOL(drm_crtc_vblank_count_and_time);
>  
> +/**
> + * drm_crtc_next_vblank_time - calculate the time of the next vblank
> + * @crtc: the crtc for which to calculate next vblank time
> + * @vblanktime: pointer to time to receive the next vblank timestamp.
> + *
> + * Calculate the expected time of the next vblank based on time of previous
> + * vblank and frame duration
> + */
> +int drm_crtc_next_vblank_time(struct drm_crtc *crtc, ktime_t *vblanktime)
> +{
> +	unsigned int pipe = drm_crtc_index(crtc);
> +	struct drm_vblank_crtc *vblank = &crtc->dev->vblank[pipe];
> +	u64 count;
> +
> +	if (!vblank->framedur_ns)
> +		return -EINVAL;
> +
> +	count = drm_vblank_count_and_time(crtc->dev, pipe, vblanktime);
> +
> +	/*
> +	 * If we don't get a valid count, then we probably also don't
> +	 * have a valid time:
> +	 */
> +	if (!count)
> +		return -EINVAL;
> +
> +	*vblanktime = ktime_add(*vblanktime, ns_to_ktime(vblank->framedur_ns));

I'd rather this not do any arithmetic, i.e. add, and simply return the calculated
remaining time, i.e. time left--so instead of add, it would simply assign
the remaining time, and possibly rename the vblanktime to something like "time_to_vblank."

Changing the top comment to "calculate the time remaining to the next vblank".
-- 
Regards,
Luben

> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_crtc_next_vblank_time);
> +
>  static void send_vblank_event(struct drm_device *dev,
>  		struct drm_pending_vblank_event *e,
>  		u64 seq, ktime_t now)
> diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
> index 733a3e2d1d10..a63bc2c92f3c 100644
> --- a/include/drm/drm_vblank.h
> +++ b/include/drm/drm_vblank.h
> @@ -230,6 +230,7 @@ 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);
> +int drm_crtc_next_vblank_time(struct drm_crtc *crtc, ktime_t *vblanktime);
>  void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
>  			       struct drm_pending_vblank_event *e);
>  void drm_crtc_arm_vblank_event(struct drm_crtc *crtc,