[PATCH 01/19] drm/core: Add drm_accurate_vblank_count, v5.

[Mario Kleiner]

On 04/25/2016 08:32 AM, 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 at gmail.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com> #v4
> Acked-by: David Airlie <airlied at linux.ie> #irc, v4
> ---
>
> Unfortunately WARN_ON(!dev->disable_vblank_immediate) doesn't work on gen2,
> which is the reason this function is created. So I used
> WARN_ON(!get_vblank_timestamp) instead.

That's a weaker warning. I'd like to have the WARN_ON and the doc text 
to be more frightening/restrictive to discourage abuse.

But can't you simply remove that !IS_GEN2 check now and always set 
dev->disable_vblank_immediate = true? The reason for that exception was 
that GEN2 doesn't have a hw vblank counter. But it has scanout pos based 
vblank timestamping, which i'd assume is well behaved. With the new 
scanout based vblank counter emulation in drm_update_vblank_count() 
since around Linux 4.4 you therefore essentially have a proper emulated 
vblank counter, so this should be safe. Ville will probably know. 
Otherwise you couldn't trust drm_accurate_vblank_count() here, because 
it depends on the same logic, no?

-mario


>
>   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 005202ea5900..90527c41cd5a 100644
> --- a/include/drm/drmP.h
> +++ b/include/drm/drmP.h
> @@ -995,6 +995,7 @@ 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,
>