@@ -96,6 +96,20 @@ drm_simple_kms_crtc_mode_valid(struct drm_crtc *crtc,
 	return pipe->funcs->mode_valid(pipe, mode);
 }
 
+static bool
+drm_simple_kms_crtc_mode_fixup(struct drm_crtc *crtc,
+			       const struct drm_display_mode *mode,
+			       struct drm_display_mode *adjusted_mode)
+{
+	struct drm_simple_display_pipe *pipe;
+
+	pipe = container_of(crtc, struct drm_simple_display_pipe, crtc);
+	if (!pipe->funcs || !pipe->funcs->mode_fixup)
+		return true;
+
+	return pipe->funcs->mode_fixup(crtc, mode, adjusted_mode);
+}
+
 static int drm_simple_kms_crtc_check(struct drm_crtc *crtc,
 				     struct drm_atomic_state *state)
 {

@@ -141,6 +155,7 @@ static void drm_simple_kms_crtc_disable(struct drm_crtc *crtc,
 
 static const struct drm_crtc_helper_funcs drm_simple_kms_crtc_helper_funcs = {
 	.mode_valid = drm_simple_kms_crtc_mode_valid,
+	.mode_fixup = drm_simple_kms_crtc_mode_fixup,
 	.atomic_check = drm_simple_kms_crtc_check,
 	.atomic_enable = drm_simple_kms_crtc_enable,
 	.atomic_disable = drm_simple_kms_crtc_disable,

@@ -52,6 +52,49 @@ struct drm_simple_display_pipe_funcs {
 	enum drm_mode_status (*mode_valid)(struct drm_simple_display_pipe *pipe,
 					   const struct drm_display_mode *mode);
 
+	/**
+	 * @mode_fixup:
+	 *
+	 * This callback is used to validate a mode. The parameter mode is the
+	 * display mode that userspace requested, adjusted_mode is the mode the
+	 * encoders need to be fed with. Note that this is the inverse semantics
+	 * of the meaning for the &drm_encoder and &drm_bridge_funcs.mode_fixup
+	 * vfunc. If the CRTC of the simple display pipe cannot support the
+	 * requested conversion from mode to adjusted_mode it should reject the
+	 * modeset.
+	 *
+	 * This function is optional.
+	 *
+	 * NOTE:
+	 *
+	 * This function is called in the check phase of atomic modesets, which
+	 * can be aborted for any reason (including on userspace's request to
+	 * just check whether a configuration would be possible). Atomic drivers
+	 * MUST NOT touch any persistent state (hardware or software) or data
+	 * structures except the passed in adjusted_mode parameter.
+	 *
+	 * Atomic drivers which need to inspect and adjust more state should
+	 * instead use the @atomic_check callback, but note that they're not
+	 * perfectly equivalent: @mode_valid is called from
+	 * drm_atomic_helper_check_modeset(), but @atomic_check is called from
+	 * drm_atomic_helper_check_planes(), because originally it was meant for
+	 * plane update checks only.
+	 *
+	 * Also beware that userspace can request its own custom modes, neither
+	 * core nor helpers filter modes to the list of probe modes reported by
+	 * the GETCONNECTOR IOCTL and stored in &drm_connector.modes. To ensure
+	 * that modes are filtered consistently put any CRTC constraints and
+	 * limits checks into @mode_valid.
+	 *
+	 * RETURNS:
+	 *
+	 * True if an acceptable configuration is possible, false if the modeset
+	 * operation should be rejected.
+	 */
+	bool (*mode_fixup)(struct drm_crtc *crtc,
+			   const struct drm_display_mode *mode,
+			   struct drm_display_mode *adjusted_mode);
+
 	/**
 	 * @enable:
 	 *