[RFC v1 3/6] drm: Add a capability flag to support additional flip completion signalling

[Vivek Kasireddy]

If a driver supports this capability, it means that there would be an
additional signalling mechanism for a page flip completion in addition
to out_fence or DRM_MODE_PAGE_FLIP_EVENT.

This capability may only be relevant for Virtual KMS drivers and is currently
used only by virtio-gpu. Also, it can provide a potential solution for:
https://gitlab.freedesktop.org/wayland/weston/-/issues/514

Signed-off-by: Vivek Kasireddy <vivek.kasireddy at intel.com>
---
 drivers/gpu/drm/drm_ioctl.c   | 3 +++
 include/drm/drm_mode_config.h | 8 ++++++++
 include/uapi/drm/drm.h        | 1 +
 3 files changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 8b8744dcf691..8a420844f8bc 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -302,6 +302,9 @@ static int drm_getcap(struct drm_device *dev, void *data, struct drm_file *file_
 	case DRM_CAP_CRTC_IN_VBLANK_EVENT:
 		req->value = 1;
 		break;
+	case DRM_CAP_RELEASE_FENCE:
+		req->value = dev->mode_config.release_fence;
+		break;
 	default:
 		return -EINVAL;
 	}
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 12b964540069..944bebf359d7 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -935,6 +935,14 @@ struct drm_mode_config {
 	 */
 	bool normalize_zpos;
 
+	/**
+	 * @release_fence:
+	 *
+	 * If this option is set, it means there would be an additional signalling
+	 * mechanism for a page flip completion.
+	 */
+	bool release_fence;
+
 	/**
 	 * @modifiers_property: Plane property to list support modifier/format
 	 * combination.
diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
index 3b810b53ba8b..8b8985f65581 100644
--- a/include/uapi/drm/drm.h
+++ b/include/uapi/drm/drm.h
@@ -767,6 +767,7 @@ struct drm_gem_open {
  * Documentation/gpu/drm-mm.rst, section "DRM Sync Objects".
  */
 #define DRM_CAP_SYNCOBJ_TIMELINE	0x14
+#define DRM_CAP_RELEASE_FENCE		0x15
 
 /* DRM_IOCTL_GET_CAP ioctl argument type */
 struct drm_get_cap {
-- 
2.30.2