[PATCH] drm/amd/display: Don't set mode_changed=false if the stream was removed

[Nicholas Kazlauskas]

[Why]
When switching from vt to desktop with EDID emulation we can receive
an atomic commit such that we have a crtc where mode_changed = true.

During the dm_update_crtc_state disable pass we remove the stream from
the context and free it on the dm_new_crtc_state.

During the enable pass we compare the new provisional stream to the
dm_old_crtc_state->stream and determine that the stream is unchanged
and no scaling has been changed.

Following this, new_crtc_state->mode_changed is then set to false.
The connectors haven't changed and the CRTC active state hasn't changed
so drm_atomic_crtc_needs_modeset returns false, so we jump to
skip_modeset and we hit:

BUG_ON(dm_new_crtc_state->stream == NULL);

...since the old stream is gone from the context and the new stream is
also still NULL.

[How]
Ensure that we still a stream to reuse before checking if we can reuse
the old stream without a full modeset.

Cc: Roman Li <Roman.Li at amd.com>
Cc: Leo Li <sunpeng.li at amd.com>
Cc: Harry Wentland <Harry.Wentland at amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 57359037ed7c..796f83ca7a4c 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -6248,7 +6248,17 @@ static int dm_update_crtc_state(struct amdgpu_display_manager *dm,
 
 		dm_new_crtc_state->abm_level = dm_new_conn_state->abm_level;
 
-		if (dc_is_stream_unchanged(new_stream, dm_old_crtc_state->stream) &&
+		/*
+		 * If we already removed the old stream from the context
+		 * (and set the new stream to NULL) then we can't reuse
+		 * the old stream even if the stream and scaling are unchanged.
+		 * We'll hit the BUG_ON and black screen.
+		 *
+		 * TODO: Refactor this function to allow this check to work
+		 * in all conditions.
+		 */
+		if (dm_new_crtc_state->stream &&
+		    dc_is_stream_unchanged(new_stream, dm_old_crtc_state->stream) &&
 		    dc_is_stream_scaling_unchanged(new_stream, dm_old_crtc_state->stream)) {
 			new_crtc_state->mode_changed = false;
 			DRM_DEBUG_DRIVER("Mode change not required, setting mode_changed to %d",
-- 
2.17.1