render/drm_syncobj: introduce wlr_drm_syncobj_timeline

wlr_drm_syncobj_timeline is a synchronization primitive based on
drm_syncobj timelines. They are heavily inspired from Vulkan
timeline semaphores [1].

[1]: https://www.khronos.org/blog/vulkan-timeline-semaphores

include/wlr/render/drm_syncobj.h

@@ -0,0 +1,71 @@
+#ifndef WLR_RENDER_DRM_SYNCOBJ_H
+#define WLR_RENDER_DRM_SYNCOBJ_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+/**
+ * A synchronization timeline.
+ *
+ * Timelines are used to synchronize accesses to buffers. Given a producer
+ * (writing contents to a buffer) and a consumer (reading from the buffer), the
+ * compositor needs to synchronize back-and-forth between these two users. The
+ * consumer needs to wait for the producer to signal that they're done with the
+ * writes, and the producer needs to wait for the consumer to signal that
+ * they're done with the reads.
+ *
+ * Timelines provide synchronization points in the form of monotonically
+ * increasing 64-bit integer values.
+ *
+ * wlroots timelines are designed after Vulkan timeline semaphores. For more
+ * information on the Vulkan APIs, see:
+ * https://www.khronos.org/blog/vulkan-timeline-semaphores
+ *
+ * wlroots timelines are powered by DRM synchronization objects (drm_syncobj):
+ * https://dri.freedesktop.org/docs/drm/gpu/drm-mm.html#drm-sync-objects
+ */
+struct wlr_drm_syncobj_timeline {
+	int drm_fd;
+	uint32_t handle;
+
+	// private state
+
+	size_t n_refs;
+};
+
+/**
+ * Create a new synchronization timeline.
+ */
+struct wlr_drm_syncobj_timeline *wlr_drm_syncobj_timeline_create(int drm_fd);
+/**
+ * Reference a synchronization timeline.
+ */
+struct wlr_drm_syncobj_timeline *wlr_drm_syncobj_timeline_ref(struct wlr_drm_syncobj_timeline *timeline);
+/**
+ * Unreference a synchronization timeline.
+ */
+void wlr_drm_syncobj_timeline_unref(struct wlr_drm_syncobj_timeline *timeline);
+/**
+ * Export a timeline point as a sync_file FD.
+ *
+ * The returned sync_file will be signalled when the provided point is
+ * signalled on the timeline.
+ *
+ * This allows inter-operation with other APIs which don't support drm_syncobj
+ * yet. The synchronization point needs to have already materialized:
+ * wait-before-signal is not supported.
+ */
+int wlr_drm_syncobj_timeline_export_sync_file(struct wlr_drm_syncobj_timeline *timeline,
+	uint64_t src_point);
+/**
+ * Import a timeline point from a sync_file FD.
+ *
+ * The provided timeline point will be signalled when the provided sync_file is.
+ *
+ * This allows inter-operation with other APIs which don't support drm_syncobj
+ * yet.
+ */
+bool wlr_drm_syncobj_timeline_import_sync_file(struct wlr_drm_syncobj_timeline *timeline,
+	uint64_t dst_point, int sync_file_fd);
+
+#endif

render/drm_syncobj.c

@@ -0,0 +1,98 @@
+#include <assert.h>
+#include <xf86drm.h>
+#include <stdlib.h>
+#include <wlr/render/drm_syncobj.h>
+#include <wlr/util/log.h>
+
+struct wlr_drm_syncobj_timeline *wlr_drm_syncobj_timeline_create(int drm_fd) {
+	struct wlr_drm_syncobj_timeline *timeline = calloc(1, sizeof(*timeline));
+	if (timeline == NULL) {
+		return NULL;
+	}
+	timeline->drm_fd = drm_fd;
+	timeline->n_refs = 1;
+
+	if (drmSyncobjCreate(drm_fd, 0, &timeline->handle) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjCreate failed");
+		free(timeline);
+		return NULL;
+	}
+
+	return timeline;
+}
+
+struct wlr_drm_syncobj_timeline *wlr_drm_syncobj_timeline_ref(struct wlr_drm_syncobj_timeline *timeline) {
+	timeline->n_refs++;
+	return timeline;
+}
+
+void wlr_drm_syncobj_timeline_unref(struct wlr_drm_syncobj_timeline *timeline) {
+	if (timeline == NULL) {
+		return;
+	}
+
+	assert(timeline->n_refs > 0);
+	timeline->n_refs--;
+	if (timeline->n_refs > 0) {
+		return;
+	}
+
+	drmSyncobjDestroy(timeline->drm_fd, timeline->handle);
+	free(timeline);
+}
+
+int wlr_drm_syncobj_timeline_export_sync_file(struct wlr_drm_syncobj_timeline *timeline,
+		uint64_t src_point) {
+	int sync_file_fd = -1;
+
+	uint32_t syncobj_handle;
+	if (drmSyncobjCreate(timeline->drm_fd, 0, &syncobj_handle) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjCreate failed");
+		return -1;
+	}
+
+	if (drmSyncobjTransfer(timeline->drm_fd, syncobj_handle, 0,
+			timeline->handle, src_point, 0) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjTransfer failed");
+		goto out;
+	}
+
+	if (drmSyncobjExportSyncFile(timeline->drm_fd,
+			syncobj_handle, &sync_file_fd) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjExportSyncFile failed");
+		goto out;
+	}
+
+out:
+	drmSyncobjDestroy(timeline->drm_fd, syncobj_handle);
+	return sync_file_fd;
+}
+
+bool wlr_drm_syncobj_timeline_import_sync_file(struct wlr_drm_syncobj_timeline *timeline,
+		uint64_t dst_point, int sync_file_fd) {
+	bool ok = false;
+
+	uint32_t syncobj_handle;
+	if (drmSyncobjCreate(timeline->drm_fd, 0, &syncobj_handle) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjCreate failed");
+		return -1;
+	}
+
+	if (drmSyncobjImportSyncFile(timeline->drm_fd, syncobj_handle,
+			sync_file_fd) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjImportSyncFile failed");
+		goto out;
+	}
+
+	if (drmSyncobjTransfer(timeline->drm_fd, timeline->handle, dst_point,
+			syncobj_handle, 0, 0) != 0) {
+		wlr_log_errno(WLR_ERROR, "drmSyncobjTransfer failed");
+		goto out;
+	}
+
+	ok = true;
+
+out:
+	drmSyncobjDestroy(timeline->drm_fd, syncobj_handle);
+	return ok;
+}

render/meson.build

@@ -9,6 +9,7 @@ wlr_files += files(
 	'color.c',
 	'dmabuf.c',
 	'drm_format_set.c',
+	'drm_syncobj.c',
 	'pass.c',
 	'pixel_format.c',
 	'swapchain.c',