[PATCH] drm: atomic helper: Add special quirk tail function

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

This adds (yet another) variant of the
drm_atomic_helper_commit_tail() helper function to deal
with regressions caused by reordering the bridge
prepare/enablement sequence.

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.

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().

This patch from Marek Vasut:
https://lore.kernel.org/all/20251107230517.471894-1-marek.vasut%2Brenesas%40mailbox.org/
solves part of the problem for drivers using custom
tail functions, since MCDE is using helpers only, we
add a new helper function that exploits the new
drm_atomic_helper_commit_modeset_enables_crtc_early()
and use that in MCDE.

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>
---
This obviously needs Marek's patch to be applied first, as
it is a prerequisite.
---
 drivers/gpu/drm/drm_atomic_helper.c | 32 ++++++++++++++++++++++++++++++++
 drivers/gpu/drm/mcde/mcde_drv.c     |  5 +++--
 include/drm/drm_atomic_helper.h     |  1 +
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index f03b93c72b8f..fe30159d13b1 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1976,6 +1976,38 @@ void drm_atomic_helper_commit_tail_rpm(struct drm_atomic_state *state)
 }
 EXPORT_SYMBOL(drm_atomic_helper_commit_tail_rpm);
 
+/**
+ * drm_atomic_helper_commit_tail_early - commit atomic update to hardware
+ * @state: new modeset state to be committed
+ *
+ * This is an alternative implementation for the
+ * &drm_mode_config_helper_funcs.atomic_commit_tail hook, for drivers
+ * that support runtime_pm or need the CRTC to be enabled to perform a
+ * commit, and also need the CRTC to be enabled before preparing any
+ * bridhes. Otherwise, one should use the default implementation
+ * drm_atomic_helper_commit_tail().
+ */
+void drm_atomic_helper_commit_tail_early(struct drm_atomic_state *state)
+{
+	struct drm_device *dev = state->dev;
+
+	drm_atomic_helper_commit_modeset_disables(dev, state);
+
+	drm_atomic_helper_commit_modeset_enables_crtc_early(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);
+}
+EXPORT_SYMBOL(drm_atomic_helper_commit_tail_early);
+
 static void commit_tail(struct drm_atomic_state *state)
 {
 	struct drm_device *dev = state->dev;
diff --git a/drivers/gpu/drm/mcde/mcde_drv.c b/drivers/gpu/drm/mcde/mcde_drv.c
index 5f2c462bad7e..4d88b342ed9e 100644
--- a/drivers/gpu/drm/mcde/mcde_drv.c
+++ b/drivers/gpu/drm/mcde/mcde_drv.c
@@ -104,9 +104,10 @@ 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.
+	 * the case with e.g. DSI displays, and also make sure that the
+	 * CRTC is enabled before any bridges are prepared.
 	 */
-	.atomic_commit_tail = drm_atomic_helper_commit_tail_rpm,
+	.atomic_commit_tail = drm_atomic_helper_commit_tail_early,
 };
 
 static irqreturn_t mcde_irq(int irq, void *data)
diff --git a/include/drm/drm_atomic_helper.h b/include/drm/drm_atomic_helper.h
index d7fb473db343..75e480760313 100644
--- a/include/drm/drm_atomic_helper.h
+++ b/include/drm/drm_atomic_helper.h
@@ -64,6 +64,7 @@ int drm_atomic_helper_check(struct drm_device *dev,
 			    struct drm_atomic_state *state);
 void drm_atomic_helper_commit_tail(struct drm_atomic_state *state);
 void drm_atomic_helper_commit_tail_rpm(struct drm_atomic_state *state);
+void drm_atomic_helper_commit_tail_early(struct drm_atomic_state *state);
 int drm_atomic_helper_commit(struct drm_device *dev,
 			     struct drm_atomic_state *state,
 			     bool nonblock);

---
base-commit: 643e6ae9e34fc3a87c7953b6929265e93b9c38e1
change-id: 20251118-mcde-drm-regression-33deb78a968f

Best regards,
-- 
Linus Walleij <linus.walleij@linaro.org>