drm/i915: Inifite timeout for wait ioctl

Change the ns_timeout parameter of the wait ioctl to a signed value.
Doing this allows the kernel to provide an infinite wait when a timeout
of less than 0 is provided. This mimics select/poll.

Initially the parameter was meant to match up with the GL spec 1:1, but
after being made aware of how much 2^64 - 1 nanoseconds actually is, I
do not think anyone will ever notice the loss of 1 bit.

The infinite timeout on waiting is similar to the existing i915
userspace interface with the exception that struct_mutex is dropped
while doing the wait in this ioctl.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2082,11 +2082,14 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	struct drm_i915_gem_wait *args = data;
 	struct drm_i915_gem_object *obj;
 	struct intel_ring_buffer *ring = NULL;
-	struct timespec timeout;
+	struct timespec timeout_stack, *timeout = NULL;
 	u32 seqno = 0;
 	int ret = 0;
 
-	timeout = ns_to_timespec(args->timeout_ns);
+	if (args->timeout_ns >= 0) {
+		timeout_stack = ns_to_timespec(args->timeout_ns);
+		timeout = &timeout_stack;
+	}
 
 	ret = i915_mutex_lock_interruptible(dev);
 	if (ret)
@@ -2122,9 +2125,11 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	drm_gem_object_unreference(&obj->base);
 	mutex_unlock(&dev->struct_mutex);
 
-	ret = __wait_seqno(ring, seqno, true, &timeout);
-	WARN_ON(!timespec_valid(&timeout));
-	args->timeout_ns = timespec_to_ns(&timeout);
+	ret = __wait_seqno(ring, seqno, true, timeout);
+	if (timeout) {
+		WARN_ON(!timespec_valid(timeout));
+		args->timeout_ns = timespec_to_ns(timeout);
+	}
 	return ret;
 
 out:

include/drm/i915_drm.h

@@ -893,7 +893,7 @@ struct drm_i915_gem_wait {
 	__u32 bo_handle;
 	__u32 flags;
 	/** Number of nanoseconds to wait, Returns time remaining. */
-	__u64 timeout_ns;
+	__s64 timeout_ns;
 };
 
 #endif				/* _I915_DRM_H_ */