drm/i915: Use vblank workers for gamma updates

The pipe gamma registers are single buffered so they should only
be updated during the vblank to avoid screen tearing. In fact they
really should only be updated between start of vblank and frame
start because that is the only time the pipe is guaranteed to be
empty. Already at frame start the pipe begins to fill up with
data for the next frame.

Unfortunately frame start happens ~1 scanline after the start
of vblank which in practice doesn't always leave us enough time to
finish the gamma update in time (gamma LUTs can be several KiB of
data we have to bash into the registers). However we must try our
best and so we'll add a vblank work for each pipe from where we
can do the gamma update. Additionally we could consider pushing
frame start forward to the max of ~4 scanlines after start of
vblank. But not sure that's exactly a validated configuration.
As it stands the ~100 first pixels tend to make it through with
the old gamma values.

Even though the vblank worker is running on a high prority thread
we still have to contend with C-states. If the CPU happens be in
a deep C-state when the vblank interrupt arrives even the irq
handler gets delayed massively (I've observed dozens of scanlines
worth of latency). To avoid that problem we'll use the qos mechanism
to keep the CPU awake while the vblank work is scheduled.

With all this hooked up we can finally enjoy near atomic gamma
updates. It even works across several pipes from the same atomic
commit which previously was a total fail because we did the
gamma updates for each pipe serially after waiting for all
pipes to have latched the double buffered registers.

In the future the DSB should take over this responsibility
which will hopefully avoid some of these issues.

Kudos to Lyude for finishing the actual vblank workers.
Works like the proverbial train toilet.

v2: Add missing intel_atomic_state fwd declaration
v3: Clean up properly when not scheduling the worker
v4: Clean up the rest and add tracepoints
v5: s/intel_wait_for_vblank_works/intel_wait_for_vblank_workers/ (Jani,Uma)

CC: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020223339.669-4-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>

drivers/gpu/drm/i915/display/intel_crtc.c

@@ -3,12 +3,14 @@
  * Copyright © 2020 Intel Corporation
  */
 #include <linux/kernel.h>
+#include <linux/pm_qos.h>
 #include <linux/slab.h>
 
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_plane.h>
 #include <drm/drm_plane_helper.h>
+#include <drm/drm_vblank_work.h>
 
 #include "i915_trace.h"
 #include "i915_vgpu.h"
@@ -167,6 +169,8 @@ static void intel_crtc_destroy(struct drm_crtc *_crtc)
 {
 	struct intel_crtc *crtc = to_intel_crtc(_crtc);
 
+	cpu_latency_qos_remove_request(&crtc->vblank_pm_qos);
+
 	drm_crtc_cleanup(&crtc->base);
 	kfree(crtc);
 }
@@ -344,6 +348,8 @@ int intel_crtc_init(struct drm_i915_private *dev_priv, enum pipe pipe)
 
 	intel_crtc_crc_init(crtc);
 
+	cpu_latency_qos_add_request(&crtc->vblank_pm_qos, PM_QOS_DEFAULT_VALUE);
+
 	drm_WARN_ON(&dev_priv->drm, drm_crtc_index(&crtc->base) != crtc->pipe);
 
 	return 0;
@@ -354,6 +360,65 @@ int intel_crtc_init(struct drm_i915_private *dev_priv, enum pipe pipe)
 	return ret;
 }
 
+static bool intel_crtc_needs_vblank_work(const struct intel_crtc_state *crtc_state)
+{
+	return crtc_state->hw.active &&
+		!intel_crtc_needs_modeset(crtc_state) &&
+		!crtc_state->preload_luts &&
+		(crtc_state->uapi.color_mgmt_changed ||
+		 crtc_state->update_pipe);
+}
+
+static void intel_crtc_vblank_work(struct kthread_work *base)
+{
+	struct drm_vblank_work *work = to_drm_vblank_work(base);
+	struct intel_crtc_state *crtc_state =
+		container_of(work, typeof(*crtc_state), vblank_work);
+	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
+
+	trace_intel_crtc_vblank_work_start(crtc);
+
+	intel_color_load_luts(crtc_state);
+
+	if (crtc_state->uapi.event) {
+		spin_lock_irq(&crtc->base.dev->event_lock);
+		drm_crtc_send_vblank_event(&crtc->base, crtc_state->uapi.event);
+		crtc_state->uapi.event = NULL;
+		spin_unlock_irq(&crtc->base.dev->event_lock);
+	}
+
+	trace_intel_crtc_vblank_work_end(crtc);
+}
+
+static void intel_crtc_vblank_work_init(struct intel_crtc_state *crtc_state)
+{
+	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
+
+	drm_vblank_work_init(&crtc_state->vblank_work, &crtc->base,
+			     intel_crtc_vblank_work);
+	/*
+	 * Interrupt latency is critical for getting the vblank
+	 * work executed as early as possible during the vblank.
+	 */
+	cpu_latency_qos_update_request(&crtc->vblank_pm_qos, 0);
+}
+
+void intel_wait_for_vblank_workers(struct intel_atomic_state *state)
+{
+	struct intel_crtc_state *crtc_state;
+	struct intel_crtc *crtc;
+	int i;
+
+	for_each_new_intel_crtc_in_state(state, crtc, crtc_state, i) {
+		if (!intel_crtc_needs_vblank_work(crtc_state))
+			continue;
+
+		drm_vblank_work_flush(&crtc_state->vblank_work);
+		cpu_latency_qos_update_request(&crtc->vblank_pm_qos,
+					       PM_QOS_DEFAULT_VALUE);
+	}
+}
+
 int intel_usecs_to_scanlines(const struct drm_display_mode *adjusted_mode,
 			     int usecs)
 {
@@ -387,7 +452,7 @@ static int intel_mode_vblank_start(const struct drm_display_mode *mode)
  * until a subsequent call to intel_pipe_update_end(). That is done to
  * avoid random delays.
  */
-void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
+void intel_pipe_update_start(struct intel_crtc_state *new_crtc_state)
 {
 	struct intel_crtc *crtc = to_intel_crtc(new_crtc_state->uapi.crtc);
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
@@ -402,6 +467,9 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
 	if (new_crtc_state->uapi.async_flip)
 		return;
 
+	if (intel_crtc_needs_vblank_work(new_crtc_state))
+		intel_crtc_vblank_work_init(new_crtc_state);
+
 	if (new_crtc_state->vrr.enable)
 		vblank_start = intel_vrr_vmax_vblank_start(new_crtc_state);
 	else
@@ -557,7 +625,11 @@ void intel_pipe_update_end(struct intel_crtc_state *new_crtc_state)
 	 * Would be slightly nice to just grab the vblank count and arm the
 	 * event outside of the critical section - the spinlock might spin for a
 	 * while ... */
-	if (new_crtc_state->uapi.event) {
+	if (intel_crtc_needs_vblank_work(new_crtc_state)) {
+		drm_vblank_work_schedule(&new_crtc_state->vblank_work,
+					 drm_crtc_accurate_vblank_count(&crtc->base) + 1,
+					 false);
+	} else if (new_crtc_state->uapi.event) {
 		drm_WARN_ON(&dev_priv->drm,
 			    drm_crtc_vblank_get(&crtc->base) != 0);
 

drivers/gpu/drm/i915/display/intel_crtc.h

@@ -11,6 +11,7 @@
 enum pipe;
 struct drm_display_mode;
 struct drm_i915_private;
+struct intel_atomic_state;
 struct intel_crtc;
 struct intel_crtc_state;
 
@@ -24,7 +25,8 @@ void intel_crtc_state_reset(struct intel_crtc_state *crtc_state,
 u32 intel_crtc_get_vblank_counter(struct intel_crtc *crtc);
 void intel_crtc_vblank_on(const struct intel_crtc_state *crtc_state);
 void intel_crtc_vblank_off(const struct intel_crtc_state *crtc_state);
-void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state);
+void intel_pipe_update_start(struct intel_crtc_state *new_crtc_state);
 void intel_pipe_update_end(struct intel_crtc_state *new_crtc_state);
+void intel_wait_for_vblank_workers(struct intel_atomic_state *state);
 
 #endif

drivers/gpu/drm/i915/display/intel_display.c

@@ -8689,6 +8689,8 @@ static void intel_atomic_commit_tail(struct intel_atomic_state *state)
 		intel_set_cdclk_post_plane_update(state);
 	}
 
+	intel_wait_for_vblank_workers(state);
+
 	/* FIXME: We should call drm_atomic_helper_commit_hw_done() here
 	 * already, but still need the state for the delayed optimization. To
 	 * fix this:
@@ -8703,13 +8705,6 @@ static void intel_atomic_commit_tail(struct intel_atomic_state *state)
 	for_each_new_intel_crtc_in_state(state, crtc, new_crtc_state, i) {
 		if (new_crtc_state->uapi.async_flip)
 			intel_crtc_disable_flip_done(state, crtc);
-
-		if (new_crtc_state->hw.active &&
-		    !intel_crtc_needs_modeset(new_crtc_state) &&
-		    !new_crtc_state->preload_luts &&
-		    (new_crtc_state->uapi.color_mgmt_changed ||
-		     new_crtc_state->update_pipe))
-			intel_color_load_luts(new_crtc_state);
 	}
 
 	/*

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -28,6 +28,7 @@
 
 #include <linux/async.h>
 #include <linux/i2c.h>
+#include <linux/pm_qos.h>
 #include <linux/pwm.h>
 #include <linux/sched/clock.h>
 
@@ -41,6 +42,7 @@
 #include <drm/drm_probe_helper.h>
 #include <drm/drm_rect.h>
 #include <drm/drm_vblank.h>
+#include <drm/drm_vblank_work.h>
 #include <drm/i915_mei_hdcp_interface.h>
 #include <media/cec-notifier.h>
 
@@ -1237,6 +1239,9 @@ struct intel_crtc_state {
 		u8 link_count;
 		u8 pixel_overlap;
 	} splitter;
+
+	/* for loading single buffered registers during vblank */
+	struct drm_vblank_work vblank_work;
 };
 
 enum intel_pipe_crc_source {
@@ -1321,6 +1326,9 @@ struct intel_crtc {
 	/* scalers available on this crtc */
 	int num_scalers;
 
+	/* for loading single buffered registers during vblank */
+	struct pm_qos_request vblank_pm_qos;
+
 #ifdef CONFIG_DEBUG_FS
 	struct intel_pipe_crc pipe_crc;
 #endif

drivers/gpu/drm/i915/i915_trace.h

@@ -433,6 +433,48 @@ TRACE_EVENT(intel_fbc_nuke,
 
 /* pipe updates */
 
+TRACE_EVENT(intel_crtc_vblank_work_start,
+	    TP_PROTO(struct intel_crtc *crtc),
+	    TP_ARGS(crtc),
+
+	    TP_STRUCT__entry(
+			     __field(enum pipe, pipe)
+			     __field(u32, frame)
+			     __field(u32, scanline)
+			     ),
+
+	    TP_fast_assign(
+			   __entry->pipe = crtc->pipe;
+			   __entry->frame = intel_crtc_get_vblank_counter(crtc);
+			   __entry->scanline = intel_get_crtc_scanline(crtc);
+			   ),
+
+	    TP_printk("pipe %c, frame=%u, scanline=%u",
+		      pipe_name(__entry->pipe), __entry->frame,
+		       __entry->scanline)
+);
+
+TRACE_EVENT(intel_crtc_vblank_work_end,
+	    TP_PROTO(struct intel_crtc *crtc),
+	    TP_ARGS(crtc),
+
+	    TP_STRUCT__entry(
+			     __field(enum pipe, pipe)
+			     __field(u32, frame)
+			     __field(u32, scanline)
+			     ),
+
+	    TP_fast_assign(
+			   __entry->pipe = crtc->pipe;
+			   __entry->frame = intel_crtc_get_vblank_counter(crtc);
+			   __entry->scanline = intel_get_crtc_scanline(crtc);
+			   ),
+
+	    TP_printk("pipe %c, frame=%u, scanline=%u",
+		      pipe_name(__entry->pipe), __entry->frame,
+		       __entry->scanline)
+);
+
 TRACE_EVENT(intel_pipe_update_start,
 	    TP_PROTO(struct intel_crtc *crtc),
 	    TP_ARGS(crtc),