drm/i915/fbc: disable FBC on FIFO underruns

Ever since I started working on FBC I was already aware that FBC can
really amplify the FIFO underrun symptoms. On systems where FIFO
underruns were harmless error messages, enabling FBC would cause the
underruns to give black screens.

We recently tried to enable FBC on Haswell and got reports of a system
that would hang after some hours of uptime, and the first bad commit
was the one that enabled FBC. We also observed that this system had
FIFO underrun error messages on its dmesg. Although we don't have any
evidence that fixing the underruns would solve the bug and make FBC
work properly on this machine, IMHO it's better if we minimize the
amount of possible problems by just giving up FBC whenever we detect
an underrun.

v2: New version, different implementation and commit message.
v3: Clarify the fact that we run from an IRQ handler (Chris).
v4: Also add the underrun_detected check at can_choose() to avoid
    misleading dmesg messages (DK).
v5: Fix Engrish, use READ_ONCE on the unlocked read (Chris).

Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: Lyude <cpaul@redhat.com>
Cc: stevenhoneyman@gmail.com <stevenhoneyman@gmail.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NDhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473773937-19758-1-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -973,6 +973,9 @@ struct intel_fbc {
 	bool enabled;
 	bool active;
 
+	bool underrun_detected;
+	struct work_struct underrun_work;
+
 	struct intel_fbc_state_cache {
 		struct {
 			unsigned int mode_flags;

drivers/gpu/drm/i915/intel_drv.h

@@ -1516,6 +1516,7 @@ void intel_fbc_invalidate(struct drm_i915_private *dev_priv,
 void intel_fbc_flush(struct drm_i915_private *dev_priv,
 		     unsigned int frontbuffer_bits, enum fb_op_origin origin);
 void intel_fbc_cleanup_cfb(struct drm_i915_private *dev_priv);
+void intel_fbc_handle_fifo_underrun_irq(struct drm_i915_private *dev_priv);
 
 /* intel_hdmi.c */
 void intel_hdmi_init(struct drm_device *dev, i915_reg_t hdmi_reg, enum port port);

drivers/gpu/drm/i915/intel_fbc.c

@@ -774,6 +774,14 @@ static bool intel_fbc_can_activate(struct intel_crtc *crtc)
 	struct intel_fbc *fbc = &dev_priv->fbc;
 	struct intel_fbc_state_cache *cache = &fbc->state_cache;
 
+	/* We don't need to use a state cache here since this information is
+	 * global for all CRTC.
+	 */
+	if (fbc->underrun_detected) {
+		fbc->no_fbc_reason = "underrun detected";
+		return false;
+	}
+
 	if (!cache->plane.visible) {
 		fbc->no_fbc_reason = "primary plane not visible";
 		return false;
@@ -859,6 +867,11 @@ static bool intel_fbc_can_choose(struct intel_crtc *crtc)
 		return false;
 	}
 
+	if (fbc->underrun_detected) {
+		fbc->no_fbc_reason = "underrun detected";
+		return false;
+	}
+
 	if (fbc_on_pipe_a_only(dev_priv) && crtc->pipe != PIPE_A) {
 		fbc->no_fbc_reason = "no enabled pipes can have FBC";
 		return false;
@@ -1221,6 +1234,59 @@ void intel_fbc_global_disable(struct drm_i915_private *dev_priv)
 	cancel_work_sync(&fbc->work.work);
 }
 
+static void intel_fbc_underrun_work_fn(struct work_struct *work)
+{
+	struct drm_i915_private *dev_priv =
+		container_of(work, struct drm_i915_private, fbc.underrun_work);
+	struct intel_fbc *fbc = &dev_priv->fbc;
+
+	mutex_lock(&fbc->lock);
+
+	/* Maybe we were scheduled twice. */
+	if (fbc->underrun_detected)
+		goto out;
+
+	DRM_DEBUG_KMS("Disabling FBC due to FIFO underrun.\n");
+	fbc->underrun_detected = true;
+
+	intel_fbc_deactivate(dev_priv);
+out:
+	mutex_unlock(&fbc->lock);
+}
+
+/**
+ * intel_fbc_handle_fifo_underrun_irq - disable FBC when we get a FIFO underrun
+ * @dev_priv: i915 device instance
+ *
+ * Without FBC, most underruns are harmless and don't really cause too many
+ * problems, except for an annoying message on dmesg. With FBC, underruns can
+ * become black screens or even worse, especially when paired with bad
+ * watermarks. So in order for us to be on the safe side, completely disable FBC
+ * in case we ever detect a FIFO underrun on any pipe. An underrun on any pipe
+ * already suggests that watermarks may be bad, so try to be as safe as
+ * possible.
+ *
+ * This function is called from the IRQ handler.
+ */
+void intel_fbc_handle_fifo_underrun_irq(struct drm_i915_private *dev_priv)
+{
+	struct intel_fbc *fbc = &dev_priv->fbc;
+
+	if (!fbc_supported(dev_priv))
+		return;
+
+	/* There's no guarantee that underrun_detected won't be set to true
+	 * right after this check and before the work is scheduled, but that's
+	 * not a problem since we'll check it again under the work function
+	 * while FBC is locked. This check here is just to prevent us from
+	 * unnecessarily scheduling the work, and it relies on the fact that we
+	 * never switch underrun_detect back to false after it's true. */
+	if (READ_ONCE(fbc->underrun_detected))
+		return;
+
+	schedule_work(&fbc->underrun_work);
+}
+
 /**
  * intel_fbc_init_pipe_state - initialize FBC's CRTC visibility tracking
  * @dev_priv: i915 device instance
@@ -1292,6 +1358,7 @@ void intel_fbc_init(struct drm_i915_private *dev_priv)
 	enum pipe pipe;
 
 	INIT_WORK(&fbc->work.work, intel_fbc_work_fn);
+	INIT_WORK(&fbc->underrun_work, intel_fbc_underrun_work_fn);
 	mutex_init(&fbc->lock);
 	fbc->enabled = false;
 	fbc->active = false;

drivers/gpu/drm/i915/intel_fifo_underrun.c

@@ -372,6 +372,8 @@ void intel_cpu_fifo_underrun_irq_handler(struct drm_i915_private *dev_priv,
 	if (intel_set_cpu_fifo_underrun_reporting(dev_priv, pipe, false))
 		DRM_ERROR("CPU pipe %c FIFO underrun\n",
 			  pipe_name(pipe));
+
+	intel_fbc_handle_fifo_underrun_irq(dev_priv);
 }
 
 /**