drm: apple: reject plane commit if it will crash DCP

Owing to its origin in mobile devices and the Apple TV, DCP seems to
have been designed under the assumption that no one could possibly want
a rectangle to clip the screen. If a rectangle's bottom-right edge
clips the screen, DCP will instead try to scale the destination rectangle
to the best of its ability... until it can't anymore.

DCP is not tolerant to faults and will crash if the onscreen portion of
the framebuffer ends up smaller than 32x32, or if any dimension ends up
entirely offscreen.

Use apple_plane_atomic_check() to reject requested plane states that
could crash DCP. This is the final piece of the puzzle required to
enable preliminary support for overlay planes on Apple Silicon devices.

Signed-off-by: James Calligeros <jcalligeros99@gmail.com>

Author: chadmed

drivers/gpu/drm/apple/apple_drv.c

@@ -91,6 +91,19 @@ static int apple_plane_atomic_check(struct drm_plane *plane,
 	if (IS_ERR(crtc_state))
 		return PTR_ERR(crtc_state);
 
+	/*
+	 * DCP does not allow a surface to clip off the screen, and will crash
+	 * if any blended surface is smaller than 32x32. Reject the atomic op
+	 * if the plane will crash DCP.
+	 *
+	 * This is most pertinent to cursors. Userspace should fall back to
+	 * software cursors if the plane check is rejected.
+	 */
+	if ((new_plane_state->crtc_x + 32) > crtc_state->mode.hdisplay ||
+	     (new_plane_state->crtc_y + 32) > crtc_state->mode.vdisplay) {
+		return -EINVAL;
+	}
+
 	/*
 	 * DCP limits downscaling to 2x and upscaling to 4x. Attempting to
 	 * scale outside these bounds errors out when swapping.