drm/vc4: kms: Document the muxing corner cases

We've had a number of muxing corner-cases with specific ways to reproduce
them, so let's document them to make sure they aren't lost and introduce
regressions later on.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Reviewed-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105135656.383350-6-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_kms.c

@@ -662,6 +662,28 @@ static int vc4_load_tracker_obj_init(struct vc4_dev *vc4)
 	return drmm_add_action_or_reset(&vc4->base, vc4_load_tracker_obj_fini, NULL);
 }
 
+/*
+ * The BCM2711 HVS has up to 7 outputs 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 pool of available FIFOs. However, there are 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.
+ *
+ * - 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)
 {