drm: apple: Override drm_vblank's page flip event handling [HACK]

Since we don't init/uses drm's vblank support our page flip timestamps
are CLOCK_MONOTONIC timestamps during the event generation. Since
compositors use the timestamp to schedule their next kms commit this is
timing sensitive sop move it under the drivers control.
Take the timestamp directly in the swap_complete callback.
Framebuffer swaps are unfortunately not fast with DCP. Measured time
from swap_submit to swap_complete is ~1.5 ms for dcp and ~2.3 ms for
dcpext. This warrants further investigation. Presentation timestamps
might help if delay on dcp firmware side occurs after the actual swap.
In the meantime doctor the time stamps and move the page flip completion
up to 1 ms earler. This fixes half rate refresh on external displays
displays using dcpext.

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

drivers/gpu/drm/apple/dcp-internal.h

@@ -175,6 +175,7 @@ struct apple_dcp {
 
 	/* swap id of the last completed swap */
 	u32 last_swap_id;
+	ktime_t swap_start;
 
 	/* Current display mode */
 	bool during_modeset;
@@ -253,6 +254,8 @@ struct apple_dcp {
 	int hdmi_hpd_irq;
 };
 
+void dcp_drm_crtc_page_flip(struct apple_dcp *dcp, ktime_t now);
+
 int dcp_backlight_register(struct apple_dcp *dcp);
 int dcp_backlight_update(struct apple_dcp *dcp);
 bool dcp_has_panel(struct apple_dcp *dcp);

drivers/gpu/drm/apple/dcp.c

@@ -50,6 +50,76 @@ bool hdmi_audio;
 module_param(hdmi_audio, bool, 0644);
 MODULE_PARM_DESC(hdmi_audio, "Enable unstable HDMI audio support");
 
+/* copied and simplified from drm_vblank.c */
+static void send_vblank_event(struct drm_device *dev,
+		struct drm_pending_vblank_event *e,
+		u64 seq, ktime_t now)
+{
+	struct timespec64 tv;
+
+	if (e->event.base.type != DRM_EVENT_FLIP_COMPLETE)
+		return;
+
+	tv = ktime_to_timespec64(now);
+	e->event.vbl.sequence = seq;
+	/*
+		* e->event is a user space structure, with hardcoded unsigned
+		* 32-bit seconds/microseconds. This is safe as we always use
+		* monotonic timestamps since linux-4.15
+		*/
+	e->event.vbl.tv_sec = tv.tv_sec;
+	e->event.vbl.tv_usec = tv.tv_nsec / 1000;
+
+	/*
+	 * Use the same timestamp for any associated fence signal to avoid
+	 * mismatch in timestamps for vsync & fence events triggered by the
+	 * same HW event. Frameworks like SurfaceFlinger in Android expects the
+	 * retire-fence timestamp to match exactly with HW vsync as it uses it
+	 * for its software vsync modeling.
+	 */
+	drm_send_event_timestamp_locked(dev, &e->base, now);
+}
+
+/**
+ * dcp_crtc_send_page_flip_event - helper to send vblank event after pageflip
+ *
+ * Compensate for unknown slack between page flip and arrival of the
+ * swap_complete callback. Minimal observed duration on DCP with HDMI output
+ * was around 2.3 ms. If the fb swap was submitted closer to the expected
+ * swap_complete it gets a penalty of one frame duration. This is on the border
+ * of unreasonable considering that Apple advertises support for 240 Hz (frame
+ * duration of 4.167 ms).
+ * It is unreasonable considering kwin's kms commit scheduling. Kwin commits
+ * 1.5 ms + the mode's vblank time before the expected next page flip
+ * completion. This results in presenting at half the display's rate for HDMI
+ * outputs.
+ * This might be a difference between dcp and dcpext.
+ */
+static void dcp_crtc_send_page_flip_event(struct apple_crtc *crtc,
+					  struct drm_pending_vblank_event *e,
+					  ktime_t now, ktime_t start)
+{
+	struct drm_device *dev = crtc->base.dev;
+	u64 seq;
+	unsigned int pipe = drm_crtc_index(&crtc->base);
+	ktime_t flip;
+
+	seq = 0;
+	if (start != KTIME_MIN) {
+		s64 delta = ktime_us_delta(now, start);
+		if (delta <= 500)
+			flip = now;
+		else if (delta >= 2500)
+			flip = ktime_sub_us(now, 1000);
+		else
+			flip = ktime_sub_us(now, (delta - 500) / 2);
+	} else {
+		flip = now;
+	}
+	e->pipe = pipe;
+	send_vblank_event(dev, e, seq, flip);
+}
+
 /* HACK: moved here to avoid circular dependency between apple_drv and dcp */
 void dcp_drm_crtc_vblank(struct apple_crtc *crtc)
 {
@@ -63,6 +133,23 @@ void dcp_drm_crtc_vblank(struct apple_crtc *crtc)
 	spin_unlock_irqrestore(&crtc->base.dev->event_lock, flags);
 }
 
+void dcp_drm_crtc_page_flip(struct apple_dcp *dcp, ktime_t now)
+{
+	unsigned long flags;
+	struct apple_crtc *crtc = dcp->crtc;
+
+	spin_lock_irqsave(&crtc->base.dev->event_lock, flags);
+	if (crtc->event) {
+		if (crtc->event->event.base.type == DRM_EVENT_FLIP_COMPLETE)
+			dcp_crtc_send_page_flip_event(crtc, crtc->event, now, dcp->swap_start);
+		else
+			drm_crtc_send_vblank_event(&crtc->base, crtc->event);
+		crtc->event = NULL;
+		dcp->swap_start = KTIME_MIN;
+	}
+	spin_unlock_irqrestore(&crtc->base.dev->event_lock, flags);
+}
+
 void dcp_set_dimensions(struct apple_dcp *dcp)
 {
 	int i;

drivers/gpu/drm/apple/iomfb_template.c

@@ -120,10 +120,11 @@ static u32 dcpep_cb_zero(struct apple_dcp *dcp)
 static void dcpep_cb_swap_complete(struct apple_dcp *dcp,
 				   struct DCP_FW_NAME(dc_swap_complete_resp) *resp)
 {
+	ktime_t now = ktime_get();
 	trace_iomfb_swap_complete(dcp, resp->swap_id);
 	dcp->last_swap_id = resp->swap_id;
 
-	dcp_drm_crtc_vblank(dcp->crtc);
+	dcp_drm_crtc_page_flip(dcp, now);
 }
 
 /* special */
@@ -1124,6 +1125,7 @@ static void dcp_swapped(struct apple_dcp *dcp, void *data, void *cookie)
 		dcp_drm_crtc_vblank(dcp->crtc);
 		return;
 	}
+	dcp->swap_start = ktime_get();
 
 	while (!list_empty(&dcp->swapped_out_fbs)) {
 		struct dcp_fb_reference *entry;