[PATCH v4 2/3] drm/mcde: Create custom commit tail

[Linus Walleij <linus.walleij@linaro.org>]

commit c9b1150a68d9362a0827609fc0dc1664c0d8bfe1
"drm/atomic-helper: Re-order bridge chain pre-enable and post-disable"
caused a series of regressions in all panels that send
DSI commands in their .prepare() and .unprepare()
callbacks when used with MCDE.

As the CRTC is no longer online at bridge_pre_enable()
and gone at brige_post_disable() which maps to the panel
bridge .prepare()/.unprepare() callbacks, any CRTC that
enable/disable the DSI transmitter in it's enable/disable
callbacks will be unable to send any DSI commands in the
.prepare() and .unprepare() callbacks.

However the MCDE driver definitely need the CRTC to be
enabled during .prepare()/.unprepare().

Solve this by implementing a custom commit tail function
in the MCDE driver that always enables the CRTC first
and disables it last, using the newly exported helpers.

Link: https://lore.kernel.org/dri-devel/20251026-fix-mcde-drm-regression-v2-0-8d799e488cf9@linaro.org/
Link: https://lore.kernel.org/all/20251107230517.471894-1-marek.vasut%2Brenesas%40mailbox.org/
Fixes: c9b1150a68d9 ("drm/atomic-helper: Re-order bridge chain pre-enable and post-disable")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/gpu/drm/mcde/mcde_drv.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/mcde/mcde_drv.c b/drivers/gpu/drm/mcde/mcde_drv.c
index 5f2c462bad7e..290082c86100 100644
--- a/drivers/gpu/drm/mcde/mcde_drv.c
+++ b/drivers/gpu/drm/mcde/mcde_drv.c
@@ -100,13 +100,48 @@ static const struct drm_mode_config_funcs mcde_mode_config_funcs = {
 	.atomic_commit = drm_atomic_helper_commit,
 };
 
+/*
+ * This commit tail explicitly copies and changes the behaviour of
+ * the related core DRM atomic helper instead of trying to make
+ * the core helpers overly generic.
+ */
+static void mcde_atomic_commit_tail(struct drm_atomic_state *state)
+{
+	struct drm_device *dev = state->dev;
+
+	/* Variant of drm_atomic_helper_commit_modeset_disables() */
+	drm_encoder_bridge_disable(dev, state);
+	drm_encoder_bridge_post_disable(dev, state);
+	drm_crtc_disable(dev, state);
+	drm_atomic_helper_update_legacy_modeset_state(dev, state);
+	drm_atomic_helper_calc_timestamping_constants(state);
+	drm_crtc_set_mode(dev, state);
+
+	/* Variant of drm_atomic_helper_commit_modeset_enables() */
+	drm_crtc_enable(dev, state);
+	drm_encoder_bridge_pre_enable(dev, state);
+	drm_encoder_bridge_enable(dev, state);
+	drm_atomic_helper_commit_writebacks(dev, state);
+
+	drm_atomic_helper_commit_planes(dev, state,
+					DRM_PLANE_COMMIT_ACTIVE_ONLY);
+
+	drm_atomic_helper_fake_vblank(state);
+
+	drm_atomic_helper_commit_hw_done(state);
+
+	drm_atomic_helper_wait_for_vblanks(dev, state);
+
+	drm_atomic_helper_cleanup_planes(dev, state);
+}
+
 static const struct drm_mode_config_helper_funcs mcde_mode_config_helpers = {
 	/*
 	 * Using this function is necessary to commit atomic updates
 	 * that need the CRTC to be enabled before a commit, as is
 	 * the case with e.g. DSI displays.
 	 */
-	.atomic_commit_tail = drm_atomic_helper_commit_tail_rpm,
+	.atomic_commit_tail = mcde_atomic_commit_tail,
 };
 
 static irqreturn_t mcde_irq(int irq, void *data)

-- 
2.51.1