[Intel-gfx] [PATCH] drm/i915: Asynchronously perform the set-base for a simple modeset

[Chris Wilson]

A simple modeset, where we only wish to switch over to a new framebuffer
such as the transition from fbcon to X, takes around 30-60ms. This is
due to three factors:

1. We need to make sure the fb->obj is in the display domain, which
incurs a cache flush to ensure no dirt is left on the scanout.

2. We need to flush any pending rendering before performing the mmio
so that the frame is complete before it is shown.

3. We currently wait for the vblank after the mmio to be sure that the
old fb is no longer being shown before releasing it.

(1) can only be eliminated by userspace preparing the fb->obj in advance
to already be in the display domain. This can be done through use of the
create2 ioctl, or by reusing an existing fb->obj.

However, (2) and (3) are already solved by the existing page flip
mechanism, and it is surprisingly trivial to wire them up for use in the
set-base fast path. Though it can be argued that this represents a
subtle ABI break in that the set_config ioctl now returns before the old
framebuffer is unpinned. The danger is that userspace will start to
modify it before it is no longer being shown, however we should be able
to prevent that through proper domain tracking.

By combining all of the above, we can achieve an instaneous set_config:

[     6.601] (II) intel(0): switch to mode 2560x1440 at 60.0 on pipe 0 using DP2, position (0, 0), rotation normal
[     6.601] (II) intel(0): Setting screen physical size to 677 x 381

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_display.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 809b968..c6eea51 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -9077,10 +9077,13 @@ static int intel_crtc_set_config(struct drm_mode_set *set)
 		ret = intel_set_mode(set->crtc, set->mode,
 				     set->x, set->y, set->fb);
 	} else if (config->fb_changed) {
-		intel_crtc_wait_for_pending_flips(set->crtc);
-
-		ret = intel_pipe_set_base(set->crtc,
-					  set->x, set->y, set->fb);
+		if (to_intel_framebuffer(set->fb)->obj->ring == NULL ||
+		    save_set.x != set->x || save_set.y != set->y ||
+		    intel_crtc_page_flip(set->crtc, set->fb, NULL)) {
+			intel_crtc_wait_for_pending_flips(set->crtc);
+			ret = intel_pipe_set_base(set->crtc,
+						  set->x, set->y, set->fb);
+		}
 	}
 
 	if (ret) {
-- 
1.8.1.2