[PATCH 6/7] drm/i915: Evade the vblank on the scanline of the joiner primary

[ElXreno <elxreno@gmail.com>]

The scanline counter of a joiner secondary pipe was observed to wrap
at 8192 lines on Arrow Lake, with pipes A and B joined. Sampling
PIPEDSL of both pipes, they agree up to line 8191, which is the
largest value pipe B ever returns, while pipe A counts on up to the
vmax vtotal. From line 8192 on pipe B reads 8192 lines less than
pipe A. This comes from measurement only, I have no documentation for
it, and I could not check whether it is a property of the joiner
secondary or of that pipe.

It is harmless as long as vtotal stays below 8192, which every fixed
refresh rate mode that needs the joiner does. With VRR the vmax
vtotal can be larger, 12200 for a 3840x2400@240 mode with a 48 Hz
lower limit. The vmax vblank start at line 12160 then reads as 3968
on the secondary, vblank evasion on that pipe never sees its evasion
window, and the pipe gets armed across the vblank:

 Atomic update failure on pipe B (start=48480 end=48481) time 8 us, min 12101, max 12159, scanline start 3965, end 3969

All joined pipes run off the timings of the primary, so look at the
scanline counter of the primary when evading the vblank on a
secondary. This matters for the commits that still take the per pipe
critical sections with the joiner.

Only the evasion is changed. The vblank timestamps of a secondary
still come from its own counter, and so does the end scanline in the
message above, while the start scanline is now the primary's.

Signed-off-by: ElXreno <elxreno@gmail.com>
---
 drivers/gpu/drm/i915/display/intel_vblank.c | 12 ++++++++++--
 drivers/gpu/drm/i915/display/intel_vblank.h |  2 +-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_vblank.c b/drivers/gpu/drm/i915/display/intel_vblank.c
index fc9d1e14ec87..6654ef590da6 100644
--- a/drivers/gpu/drm/i915/display/intel_vblank.c
+++ b/drivers/gpu/drm/i915/display/intel_vblank.c
@@ -11,6 +11,7 @@
 #include "intel_color.h"
 #include "intel_crtc.h"
 #include "intel_de.h"
+#include "intel_display.h"
 #include "intel_display_jiffies.h"
 #include "intel_display_regs.h"
 #include "intel_display_types.h"
@@ -683,6 +684,13 @@ void intel_vblank_evade_init(const struct intel_crtc_state *old_crtc_state,
 	/* TODO: maybe just use the active timings here? */
 	crtc_state = pre_commit_crtc_state(old_crtc_state, new_crtc_state);
 
+	/*
+	 * The scanline counter of a joiner secondary pipe has been
+	 * seen to wrap at 8192 lines (on ARL), which the VRR vmax can
+	 * exceed. The primary runs off the same timings and counts on.
+	 */
+	evade->scanline_crtc = intel_primary_crtc(crtc_state);
+
 	adjusted_mode = &crtc_state->hw.adjusted_mode;
 
 	if (crtc->mode_flags & I915_MODE_FLAG_VRR) {
@@ -738,7 +746,7 @@ int intel_vblank_evade(struct intel_vblank_evade_ctx *evade)
 		 */
 		prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
 
-		scanline = intel_get_crtc_scanline(crtc);
+		scanline = intel_get_crtc_scanline(evade->scanline_crtc);
 		if (scanline < evade->min || scanline > evade->max)
 			break;
 
@@ -774,7 +782,7 @@ int intel_vblank_evade(struct intel_vblank_evade_ctx *evade)
 	 * FIXME figure out if BXT+ DSI suffers from this as well
 	 */
 	while (evade->need_vlv_dsi_wa && scanline == evade->vblank_start)
-		scanline = intel_get_crtc_scanline(crtc);
+		scanline = intel_get_crtc_scanline(evade->scanline_crtc);
 
 	return scanline;
 }
diff --git a/drivers/gpu/drm/i915/display/intel_vblank.h b/drivers/gpu/drm/i915/display/intel_vblank.h
index 98d04cacd65f..28b4c458fa18 100644
--- a/drivers/gpu/drm/i915/display/intel_vblank.h
+++ b/drivers/gpu/drm/i915/display/intel_vblank.h
@@ -16,7 +16,7 @@ struct intel_crtc;
 struct intel_crtc_state;
 
 struct intel_vblank_evade_ctx {
-	struct intel_crtc *crtc;
+	struct intel_crtc *crtc, *scanline_crtc;
 	int min, max, vblank_start;
 	bool need_vlv_dsi_wa;
 };
-- 
2.56.0