Re: [PATCH v2 01/21] drm/core: Add drm_accurate_vblank_count, v5.

[Mario Kleiner <mario.kleiner.de@gmail.com>]

I'm fine with it. I assume the function will only be used by kms 
drivers, whose writers probably know when it is safe to call the 
function, ie. what kind of potential quirks the kms drivers timestamping 
implementation has.

Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>

On 05/17/2016 03:07 PM, Maarten Lankhorst wrote:
> This function is useful for gen2 intel devices which have no frame
> counter, but need a way to determine the current vblank count without
> racing with the vblank interrupt handler.
>
> intel_pipe_update_start checks if no vblank interrupt will occur
> during vblank evasion, but cannot check whether the vblank handler has
> run to completion. This function uses the timestamps to determine
> when the last vblank has happened, and interpolates from there.
>
> Changes since v1:
> - Take vblank_time_lock and don't use drm_vblank_count_and_time.
> Changes since v2:
> - Don't return time of last vblank.
> Changes since v3:
> - Change pipe to unsigned int. (Ville)
> - Remove unused documentation for tv_ret. (kbuild)
> Changes since v4:
> - Add warning to docs when the function is useful.
> - Add a WARN_ON when get_vblank_timestamp is unavailable.
> - Use drm_vblank_count.
>
> Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> #v4
> Acked-by: David Airlie <airlied@linux.ie> #irc, v4
> ---
>   drivers/gpu/drm/drm_irq.c | 31 +++++++++++++++++++++++++++++++
>   include/drm/drmP.h        |  1 +
>   2 files changed, 32 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index 3c1a6f18e71c..d3124b67f4a5 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -303,6 +303,37 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
>   	store_vblank(dev, pipe, diff, &t_vblank, cur_vblank);
>   }
>
> +/**
> + * drm_accurate_vblank_count - retrieve the master vblank counter
> + * @crtc: which counter to retrieve
> + *
> + * This function is similar to @drm_crtc_vblank_count but this
> + * function interpolates to handle a race with vblank irq's.
> + *
> + * This is mostly useful for hardware that can obtain the scanout
> + * position, but doesn't have a frame counter.
> + */
> +u32 drm_accurate_vblank_count(struct drm_crtc *crtc)
> +{
> +	struct drm_device *dev = crtc->dev;
> +	unsigned int pipe = drm_crtc_index(crtc);
> +	u32 vblank;
> +	unsigned long flags;
> +
> +	WARN(!dev->driver->get_vblank_timestamp,
> +	     "This function requires support for accurate vblank timestamps.");
> +
> +	spin_lock_irqsave(&dev->vblank_time_lock, flags);
> +
> +	drm_update_vblank_count(dev, pipe, 0);
> +	vblank = drm_vblank_count(dev, pipe);
> +
> +	spin_unlock_irqrestore(&dev->vblank_time_lock, flags);
> +
> +	return vblank;
> +}
> +EXPORT_SYMBOL(drm_accurate_vblank_count);
> +
>   /*
>    * Disable vblank irq's on crtc, make sure that last vblank count
>    * of hardware and corresponding consistent software vblank counter
> diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> index 360b2a74e1ef..ed890384b938 100644
> --- a/include/drm/drmP.h
> +++ b/include/drm/drmP.h
> @@ -1002,6 +1002,7 @@ extern void drm_crtc_vblank_off(struct drm_crtc *crtc);
>   extern void drm_crtc_vblank_reset(struct drm_crtc *crtc);
>   extern void drm_crtc_vblank_on(struct drm_crtc *crtc);
>   extern void drm_vblank_cleanup(struct drm_device *dev);
> +extern u32 drm_accurate_vblank_count(struct drm_crtc *crtc);
>   extern u32 drm_vblank_no_hw_counter(struct drm_device *dev, unsigned int pipe);
>
>   extern int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
>
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