+#define NUM_OUTPUTS 6
+#define NUM_CHANNELS 3
+
+/*
+ * The BCM2711 HVS has up to 7 output connected to the pixelvalves and
+ * the TXP (and therefore all the CRTCs found on that platform).
+ *
+ * The naive (and our initial) implementation would just iterate over
+ * all the active CRTCs, try to find a suitable FIFO, and then remove it
+ * from the available FIFOs pool. However, there's a few corner cases
+ * that need to be considered:
+ *
+ * - When running in a dual-display setup (so with two CRTCs involved),
+ * we can update the state of a single CRTC (for example by changing
+ * its mode using xrandr under X11) without affecting the other. In
+ * this case, the other CRTC wouldn't be in the state at all, so we
+ * need to consider all the running CRTCs in the DRM device to assign
+ * a FIFO, not just the one in the state.
+ *
+ * - To fix the above, we can't use drm_atomic_get_crtc_state on all
+ * enabled CRTCs to pull their CRTC state into the global state, since
+ * a page flip would start considering their vblank to complete. Since
+ * we don't have a guarantee that they are actually active, that
+ * vblank might never happen, and shouldn't even be considered if we
+ * want to do a page flip on a single CRTC. That can be tested by
+ * doing a modetest -v first on HDMI1 and then on HDMI0.
+ *
+ * - Since we need the pixelvalve to be disabled and enabled back when
+ * the FIFO is changed, we should keep the FIFO assigned for as long
+ * as the CRTC is enabled, only considering it free again once that
+ * CRTC has been disabled. This can be tested by booting X11 on a
+ * single display, and changing the resolution down and then back up.
+ */
+static int vc4_pv_muxing_atomic_check(struct drm_device *dev,
+ struct drm_atomic_state *state)
+{
+ unsigned long unassigned_channels = GENMASK(NUM_CHANNELS - 1, 0);
+ struct drm_crtc_state *old_crtc_state, *new_crtc_state;
+ struct drm_crtc_state *crtc_state;
+ struct drm_crtc *crtc;
+ unsigned int i;
+
+ /*
+ * Since the HVS FIFOs are shared across all the pixelvalves and
+ * the TXP (and thus all the CRTCs), we need to pull the current
+ * state of all the enabled CRTCs so that an update to a single
+ * CRTC still keeps the previous FIFOs enabled and assigned to
+ * the same CRTCs, instead of evaluating only the CRTC being
+ * modified.
+ */
+ for_each_new_or_current_crtc_state(state, crtc, crtc_state) {
+ struct vc4_crtc_state *vc4_crtc_state;
+ if (!crtc_state->enable)
+ continue;
+
+ vc4_crtc_state = to_vc4_crtc_state(crtc_state);
+ unassigned_channels &= ~BIT(vc4_crtc_state->assigned_channel);
+ }
+
+ for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state, new_crtc_state, i) {
+ struct vc4_crtc_state *new_vc4_crtc_state =
+ to_vc4_crtc_state(new_crtc_state);
+ struct vc4_crtc *vc4_crtc = to_vc4_crtc(crtc);
+ bool is_assigned = false;
+ unsigned int channel;
+
+ if (old_crtc_state->enable && !new_crtc_state->enable)
+ new_vc4_crtc_state->assigned_channel = VC4_HVS_CHANNEL_DISABLED;
+
+ if (!new_crtc_state->enable)
+ continue;
+
+ if (new_vc4_crtc_state->assigned_channel != VC4_HVS_CHANNEL_DISABLED)
+ continue;
+
+ /*
+ * The problem we have to solve here is that we have
+ * up to 7 encoders, connected to up to 6 CRTCs.
+ *
+ * Those CRTCs, depending on the instance, can be
+ * routed to 1, 2 or 3 HVS FIFOs, and we need to set
+ * the change the muxing between FIFOs and outputs in
+ * the HVS accordingly.
+ *
+ * It would be pretty hard to come up with an
+ * algorithm that would generically solve
+ * this. However, the current routing trees we support
+ * allow us to simplify a bit the problem.
+ *
+ * Indeed, with the current supported layouts, if we
+ * try to assign in the ascending crtc index order the
+ * FIFOs, we can't fall into the situation where an
+ * earlier CRTC that had multiple routes is assigned
+ * one that was the only option for a later CRTC.
+ *
+ * If the layout changes and doesn't give us that in
+ * the future, we will need to have something smarter,
+ * but it works so far.
+ */
+ for_each_set_bit(channel, &unassigned_channels,
+ sizeof(unassigned_channels)) {
+
+ if (!(BIT(channel) & vc4_crtc->data->hvs_available_channels))
+ continue;
+
+ new_vc4_crtc_state->assigned_channel = channel;
+ unassigned_channels &= ~BIT(channel);
+ is_assigned = true;
+ break;
+ }
+
+ if (!is_assigned)
+ return -EINVAL;
+ }
+
+ return 0;
+}
+