[PATCH] drm/nouveau: Only recalculate PBN/VCPI on mode/connector changes

[Lyude Paul]

I -thought- I had fixed this entirely, but it looks like that I didn't
test this thoroughly enough as we apparently still make one big mistake
with nv50_msto_atomic_check() - we don't handle the following scenario:

* CRTC #1 has n VCPI allocated to it, is attached to connector DP-4
  which is attached to encoder #1. enabled=y active=n
* CRTC #1 is changed from DP-4 to DP-5, causing:
  * DP-4 crtc=#1→NULL (VCPI n→0)
  * DP-5 crtc=NULL→#1
  * CRTC #1 steals encoder #1 back from DP-4 and gives it to DP-5
  * CRTC #1 maintains the same mode as before, just with a different
    connector
* mode_changed=n connectors_changed=y
  (we _SHOULD_ do VCPI 0→n here, but don't)

Once the above scenario is repeated once, we'll attempt freeing VCPI
from the connector that we didn't allocate due to the connectors
changing, but the mode staying the same. Sigh.

Since nv50_msto_atomic_check() has broken a few times now, let's rethink
things a bit to be more careful: limit both VCPI/PBN allocations to
mode_changed || connectors_changed, since neither VCPI or PBN should
ever need to change outside of routing and mode changes.

Signed-off-by: Lyude Paul <lyude at redhat.com>
Reported-by: Bohdan Milar <bmilar at redhat.com>
Tested-by: Bohdan Milar <bmilar at redhat.com>
Fixes: 232c9eec417a ("drm/nouveau: Use atomic VCPI helpers for MST")
References: 412e85b60531 ("drm/nouveau: Only release VCPI slots on mode changes")
Cc: Lyude Paul <lyude at redhat.com>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: David Airlie <airlied at redhat.com>
Cc: Jerry Zuo <Jerry.Zuo at amd.com>
Cc: Harry Wentland <harry.wentland at amd.com>
Cc: Juston Li <juston.li at intel.com>
Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Karol Herbst <karolherbst at gmail.com>
Cc: Ilia Mirkin <imirkin at alum.mit.edu>
Cc: <stable at vger.kernel.org> # v5.1+
---
 drivers/gpu/drm/nouveau/dispnv50/disp.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/dispnv50/disp.c b/drivers/gpu/drm/nouveau/dispnv50/disp.c
index 126703816794..5d23ab8e4917 100644
--- a/drivers/gpu/drm/nouveau/dispnv50/disp.c
+++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c
@@ -771,16 +771,20 @@ nv50_msto_atomic_check(struct drm_encoder *encoder,
 	struct nv50_head_atom *asyh = nv50_head_atom(crtc_state);
 	int slots;
 
-	/* When restoring duplicated states, we need to make sure that the
-	 * bw remains the same and avoid recalculating it, as the connector's
-	 * bpc may have changed after the state was duplicated
-	 */
-	if (!state->duplicated)
-		asyh->dp.pbn =
-			drm_dp_calc_pbn_mode(crtc_state->adjusted_mode.clock,
-					     connector->display_info.bpc * 3);
+	if (crtc_state->mode_changed || crtc_state->connectors_changed) {
+		/*
+		 * When restoring duplicated states, we need to make sure that
+		 * the bw remains the same and avoid recalculating it, as the
+		 * connector's bpc may have changed after the state was
+		 * duplicated
+		 */
+		if (!state->duplicated) {
+			const int bpp = connector->display_info.bpc * 3;
+			const int clock = crtc_state->adjusted_mode.clock;
+
+			asyh->dp.pbn = drm_dp_calc_pbn_mode(bpp, clock);
+		}
 
-	if (crtc_state->mode_changed) {
 		slots = drm_dp_atomic_find_vcpi_slots(state, &mstm->mgr,
 						      mstc->port,
 						      asyh->dp.pbn);
-- 
2.21.0