dma-buf/sync_file: Add SET_DEADLINE ioctl

The initial purpose is for igt tests, but this would also be useful for
compositors that wait until close to vblank deadline to make decisions
about which frame to show.

The igt tests can be found at:

https://gitlab.freedesktop.org/robclark/igt-gpu-tools/-/commits/fence-deadline

v2: Clarify the timebase, add link to igt tests
v3: Use u64 value in ns to express deadline.
v4: More doc

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230823215458.203366-3-robdclark@gmail.com

include/uapi/linux/sync_file.h

@@ -76,6 +76,27 @@ struct sync_file_info {
 	__u64	sync_fence_info;
 };
 
+/**
+ * struct sync_set_deadline - SYNC_IOC_SET_DEADLINE - set a deadline hint on a fence
+ * @deadline_ns: absolute time of the deadline
+ * @pad:	must be zero
+ *
+ * Allows userspace to set a deadline on a fence, see &dma_fence_set_deadline
+ *
+ * The timebase for the deadline is CLOCK_MONOTONIC (same as vblank).  For
+ * example
+ *
+ *     clock_gettime(CLOCK_MONOTONIC, &t);
+ *     deadline_ns = (t.tv_sec * 1000000000L) + t.tv_nsec + ns_until_deadline
+ */
+struct sync_set_deadline {
+	__u64	deadline_ns;
+	/* Not strictly needed for alignment but gives some possibility
+	 * for future extension:
+	 */
+	__u64	pad;
+};
+
 #define SYNC_IOC_MAGIC		'>'
 
 /*
@@ -87,5 +108,6 @@ struct sync_file_info {
 
 #define SYNC_IOC_MERGE		_IOWR(SYNC_IOC_MAGIC, 3, struct sync_merge_data)
 #define SYNC_IOC_FILE_INFO	_IOWR(SYNC_IOC_MAGIC, 4, struct sync_file_info)
+#define SYNC_IOC_SET_DEADLINE	_IOW(SYNC_IOC_MAGIC, 5, struct sync_set_deadline)
 
 #endif /* _UAPI_LINUX_SYNC_H */