drm/vblank: Document drm_crtc_vblank_restore constraints

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.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209101523.2954281-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_vblank.c

@@ -1470,20 +1470,7 @@ 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;
@@ -1519,7 +1506,6 @@ 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.
@@ -1530,9 +1516,18 @@ 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.
  */
 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);

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,