[PATCH] drm/atomic: Take the atomic toys away from X

[Daniel Vetter <daniel.vetter@ffwll.ch>]

The -modesetting ddx has a totally broken idea of how atomic works:
- doesn't disable old connectors, assuming they get auto-disable like
  with the legacy setcrtc
- assumes ASYNC_FLIP is wired through for the atomic ioctl
- not a single call to TEST_ONLY

Iow the implementation is a 1:1 translation of legacy ioctls to
atomic, which is a) broken b) pointless.

We already have bugs in both i915 and amdgpu-DC where this prevents us
from enabling neat features.

If anyone ever cares about atomic in X we can easily add a new atomic
level (req->value == 2) for X to get back the shiny toys.

Since these broken versions of -modesetting have been shipping,
there's really no other way to get out of this bind.

v2:
- add an informational dmesg output (Rob, Ajax)
- reorder after the DRIVER_ATOMIC check to avoid useless noise (Ilia)
- allow req->value > 2 so that X can do another attempt at atomic in
  the future

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
References: https://gitlab.freedesktop.org/xorg/xserver/issues/629
References: https://gitlab.freedesktop.org/xorg/xserver/merge_requests/180
References: abbc0697d5fb ("drm/fb: revert the i915 Actually configure untiled displays from master")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v1)
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com> (v1)
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Adam Jackson <ajax@redhat.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Rob Clark <robdclark@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 drivers/gpu/drm/drm_ioctl.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 2c120c58f72d..56aa8bbb3a8c 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -336,7 +336,12 @@ drm_setclientcap(struct drm_device *dev, void *data, struct drm_file *file_priv)
 	case DRM_CLIENT_CAP_ATOMIC:
 		if (!drm_core_check_feature(dev, DRIVER_ATOMIC))
 			return -EOPNOTSUPP;
-		if (req->value > 1)
+		/* The modesetting DDX has a totally broken idea of atomic. */
+		if (strstr(current->comm, "X") && req->value == 1) {
+			pr_info("broken atomic modeset userspace detected, disabling atomic\n");
+			return -EOPNOTSUPP;
+		}
+		if (req->value > 2)
 			return -EINVAL;
 		file_priv->atomic = req->value;
 		file_priv->universal_planes = req->value;
-- 
2.23.0