[PATCH] drm/vblank: Document drm_crtc_vblank_restore constraints

[Daniel Vetter]

On Tue, Feb 09, 2021 at 05:36:07PM +0200, Ville Syrjälä wrote:
> On Tue, Feb 09, 2021 at 11:15:23AM +0100, Daniel Vetter wrote:
> > I got real badly confused when trying to review a fix from Ville for
> > this. Let's try to document better what's required for this, and check
> > the minimal settings at runtime - we can't check ofc that there's
> > indeed no races in the driver callback.
> > 
> > Also noticed that the drm_vblank_restore version is unused, so lets
> > unexport that while at it.
> > 
> > Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  drivers/gpu/drm/drm_vblank.c | 25 ++++++++++---------------
> >  include/drm/drm_vblank.h     |  1 -
> >  2 files changed, 10 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> > index c914b14cfb43..05f4d4c078fd 100644
> > --- a/drivers/gpu/drm/drm_vblank.c
> > +++ b/drivers/gpu/drm/drm_vblank.c
> > @@ -1471,20 +1471,7 @@ void drm_crtc_vblank_on(struct drm_crtc *crtc)
> >  }
> >  EXPORT_SYMBOL(drm_crtc_vblank_on);
> >  
> > -/**
> > - * drm_vblank_restore - estimate missed vblanks and update vblank count.
> > - * @dev: DRM device
> > - * @pipe: CRTC index
> > - *
> > - * Power manamement features can cause frame counter resets between vblank
> > - * disable and enable. Drivers can use this function in their
> > - * &drm_crtc_funcs.enable_vblank implementation to estimate missed vblanks since
> > - * the last &drm_crtc_funcs.disable_vblank using timestamps and update the
> > - * vblank counter.
> > - *
> > - * This function is the legacy version of drm_crtc_vblank_restore().
> > - */
> > -void drm_vblank_restore(struct drm_device *dev, unsigned int pipe)
> > +static void drm_vblank_restore(struct drm_device *dev, unsigned int pipe)
> >  {
> >  	ktime_t t_vblank;
> >  	struct drm_vblank_crtc *vblank;
> > @@ -1520,7 +1507,6 @@ void drm_vblank_restore(struct drm_device *dev, unsigned int pipe)
> >  		    diff, diff_ns, framedur_ns, cur_vblank - vblank->last);
> >  	store_vblank(dev, pipe, diff, t_vblank, cur_vblank);
> >  }
> > -EXPORT_SYMBOL(drm_vblank_restore);
> >  
> >  /**
> >   * drm_crtc_vblank_restore - estimate missed vblanks and update vblank count.
> > @@ -1531,9 +1517,18 @@ EXPORT_SYMBOL(drm_vblank_restore);
> >   * &drm_crtc_funcs.enable_vblank implementation to estimate missed vblanks since
> >   * the last &drm_crtc_funcs.disable_vblank using timestamps and update the
> >   * vblank counter.
> > + *
> > + * Note that drivers must have race-free high-precision timestamping support,
> > + * i.e.  &drm_crtc_funcs.get_vblank_timestamp must be hooked up and
> > + * &drm_driver.vblank_disable_immediate must be set to indicate the
> > + * time-stamping functions are race-free against vblank hardware counter
> > + * increments.
> 
> Looks good. Might prevent someone from shooting themselves in
> the foot.

Yeah hopefully, maybe even me :-)


> 
> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Thanks for your review, I pushed to drm-misc-next.
-Daniel

> 
> >   */
> >  void drm_crtc_vblank_restore(struct drm_crtc *crtc)
> >  {
> > +	WARN_ON_ONCE(!crtc->funcs->get_vblank_timestamp);
> > +	WARN_ON_ONCE(!crtc->dev->vblank_disable_immediate);
> > +
> >  	drm_vblank_restore(crtc->dev, drm_crtc_index(crtc));
> >  }
> >  EXPORT_SYMBOL(drm_crtc_vblank_restore);
> > diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
> > index dd125f8c766c..733a3e2d1d10 100644
> > --- a/include/drm/drm_vblank.h
> > +++ b/include/drm/drm_vblank.h
> > @@ -247,7 +247,6 @@ void drm_crtc_vblank_off(struct drm_crtc *crtc);
> >  void drm_crtc_vblank_reset(struct drm_crtc *crtc);
> >  void drm_crtc_vblank_on(struct drm_crtc *crtc);
> >  u64 drm_crtc_accurate_vblank_count(struct drm_crtc *crtc);
> > -void drm_vblank_restore(struct drm_device *dev, unsigned int pipe);
> >  void drm_crtc_vblank_restore(struct drm_crtc *crtc);
> >  
> >  void drm_calc_timestamping_constants(struct drm_crtc *crtc,
> > -- 
> > 2.30.0
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch