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@xxxxxxxxxx>
---
drivers/gpu/drm/vc4/vc4_kms.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/drivers/gpu/drm/vc4/vc4_kms.c b/drivers/gpu/drm/vc4/vc4_kms.c
index 846fe8b3cb7a..3b5e62842901 100644
--- a/drivers/gpu/drm/vc4/vc4_kms.c
+++ b/drivers/gpu/drm/vc4/vc4_kms.c
@@ -612,6 +612,28 @@ static const struct drm_private_state_funcs vc4_load_tracker_state_funcs = {
#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.
+ *
+ * - 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)
{
--
2.26.2
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