drm/vblank: Allow dynamic per-crtc max_vblank_count

From Ville Syrjala
2b4f567912ad5f6653c557e424ee1bf85d8b6266 in linux 4.19.y/4.19.73
ed20151a7699bb2c77eba3610199789a126940c4 in mainline linux

3 files changed, 85 insertions, 3 deletions

diff --git a/sys/dev/pci/drm/drm_vblank.c b/sys/dev/pci/drm/drm_vblank.c
index 422d67b1a51..f695c2344f5 100644
--- a/sys/dev/pci/drm/drm_vblank.c
+++ b/sys/dev/pci/drm/drm_vblank.c
@@ -105,13 +105,20 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe,
 	write_sequnlock(&vblank->seqlock);
 }
 
+static u32 drm_max_vblank_count(struct drm_device *dev, unsigned int pipe)
+{
+	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
+
+	return vblank->max_vblank_count ?: dev->max_vblank_count;
+}
+
 /*
  * "No hw counter" fallback implementation of .get_vblank_counter() hook,
  * if there is no useable hardware frame counter available.
  */
 static u32 drm_vblank_no_hw_counter(struct drm_device *dev, unsigned int pipe)
 {
-	WARN_ON_ONCE(dev->max_vblank_count != 0);
+	WARN_ON_ONCE(drm_max_vblank_count(dev, pipe) != 0);
 	return 0;
 }
 
@@ -198,6 +205,7 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
 	ktime_t t_vblank;
 	int count = DRM_TIMESTAMP_MAXRETRIES;
 	int framedur_ns = vblank->framedur_ns;
+	u32 max_vblank_count = drm_max_vblank_count(dev, pipe);
 
 	/*
 	 * Interrupts were disabled prior to this call, so deal with counter
@@ -216,9 +224,9 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
 		rc = drm_get_last_vbltimestamp(dev, pipe, &t_vblank, in_vblank_irq);
 	} while (cur_vblank != __get_vblank_counter(dev, pipe) && --count > 0);
 
-	if (dev->max_vblank_count != 0) {
+	if (max_vblank_count) {
 		/* trust the hw counter when it's around */
-		diff = (cur_vblank - vblank->last) & dev->max_vblank_count;
+		diff = (cur_vblank - vblank->last) & max_vblank_count;
 	} else if (rc && framedur_ns) {
 		u64 diff_ns = ktime_to_ns(ktime_sub(t_vblank, vblank->time));
 
@@ -1219,6 +1227,37 @@ void drm_crtc_vblank_reset(struct drm_crtc *crtc)
 EXPORT_SYMBOL(drm_crtc_vblank_reset);
 
 /**
+ * drm_crtc_set_max_vblank_count - configure the hw max vblank counter value
+ * @crtc: CRTC in question
+ * @max_vblank_count: max hardware vblank counter value
+ *
+ * Update the maximum hardware vblank counter value for @crtc
+ * at runtime. Useful for hardware where the operation of the
+ * hardware vblank counter depends on the currently active
+ * display configuration.
+ *
+ * For example, if the hardware vblank counter does not work
+ * when a specific connector is active the maximum can be set
+ * to zero. And when that specific connector isn't active the
+ * maximum can again be set to the appropriate non-zero value.
+ *
+ * If used, must be called before drm_vblank_on().
+ */
+void drm_crtc_set_max_vblank_count(struct drm_crtc *crtc,
+				   u32 max_vblank_count)
+{
+	struct drm_device *dev = crtc->dev;
+	unsigned int pipe = drm_crtc_index(crtc);
+	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
+
+	WARN_ON(dev->max_vblank_count);
+	WARN_ON(!READ_ONCE(vblank->inmodeset));
+
+	vblank->max_vblank_count = max_vblank_count;
+}
+EXPORT_SYMBOL(drm_crtc_set_max_vblank_count);
+
+/**
  * drm_crtc_vblank_on - enable vblank events on a CRTC
  * @crtc: CRTC in question
  *
diff --git a/sys/dev/pci/drm/include/drm/drm_device.h b/sys/dev/pci/drm/include/drm/drm_device.h
index dcfc8a5b773..97b82a0a9ee 100644
--- a/sys/dev/pci/drm/include/drm/drm_device.h
+++ b/sys/dev/pci/drm/include/drm/drm_device.h
@@ -113,6 +113,27 @@ struct drm_device {
 	struct mutex vblank_time_lock;    /**< Protects vblank count and time updates during vblank enable/disable */
 	struct mutex vbl_lock;
 
+	/**
+	 * @max_vblank_count:
+	 *
+	 * Maximum value of the vblank registers. This value +1 will result in a
+	 * wrap-around of the vblank register. It is used by the vblank core to
+	 * handle wrap-arounds.
+	 *
+	 * If set to zero the vblank core will try to guess the elapsed vblanks
+	 * between times when the vblank interrupt is disabled through
+	 * high-precision timestamps. That approach is suffering from small
+	 * races and imprecision over longer time periods, hence exposing a
+	 * hardware vblank counter is always recommended.
+	 *
+	 * This is the statically configured device wide maximum. The driver
+	 * can instead choose to use a runtime configurable per-crtc value
+	 * &drm_vblank_crtc.max_vblank_count, in which case @max_vblank_count
+	 * must be left at zero. See drm_crtc_set_max_vblank_count() on how
+	 * to use the per-crtc value.
+	 *
+	 * If non-zero, &drm_crtc_funcs.get_vblank_counter must be set.
+	 */
 	u32 max_vblank_count;           /**< size of vblank counter register */
 
 	/**
diff --git a/sys/dev/pci/drm/include/drm/drm_vblank.h b/sys/dev/pci/drm/include/drm/drm_vblank.h
index cfea10b1916..f731cca4447 100644
--- a/sys/dev/pci/drm/include/drm/drm_vblank.h
+++ b/sys/dev/pci/drm/include/drm/drm_vblank.h
@@ -129,6 +129,26 @@ struct drm_vblank_crtc {
 	 */
 	u32 last;
 	/**
+	 * @max_vblank_count:
+	 *
+	 * Maximum value of the vblank registers for this crtc. This value +1
+	 * will result in a wrap-around of the vblank register. It is used
+	 * by the vblank core to handle wrap-arounds.
+	 *
+	 * If set to zero the vblank core will try to guess the elapsed vblanks
+	 * between times when the vblank interrupt is disabled through
+	 * high-precision timestamps. That approach is suffering from small
+	 * races and imprecision over longer time periods, hence exposing a
+	 * hardware vblank counter is always recommended.
+	 *
+	 * This is the runtime configurable per-crtc maximum set through
+	 * drm_crtc_set_max_vblank_count(). If this is used the driver
+	 * must leave the device wide &drm_device.max_vblank_count at zero.
+	 *
+	 * If non-zero, &drm_crtc_funcs.get_vblank_counter must be set.
+	 */
+	u32 max_vblank_count;
+	/**
 	 * @inmodeset: Tracks whether the vblank is disabled due to a modeset.
 	 * For legacy driver bit 2 additionally tracks whether an additional
 	 * temporary vblank reference has been acquired to paper over the
@@ -206,4 +226,6 @@ bool drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
 void drm_calc_timestamping_constants(struct drm_crtc *crtc,
 				     const struct drm_display_mode *mode);
 wait_queue_head_t *drm_crtc_vblank_waitqueue(struct drm_crtc *crtc);
+void drm_crtc_set_max_vblank_count(struct drm_crtc *crtc,
+				   u32 max_vblank_count);
 #endif