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,
>
