[Intel-gfx] [PATCH] drm/i915: Extend wait-ioctl to only wait on writes

[Chris Wilson]

Currently, we allow read-read CPU/GPU concurrency via set-domain-ioctl,
but we don't have a similar facility for a plain wait-ioctl. If we add a
new flag that userspace can use to opt-in for only waiting for GPU
writes, userspace can use it to co-ordinate its own read-read
concurrency (without the side-effect of touching cache domains on the
GEM object).

To test whether this flag is available, userspace needs to do a query
operation like:

bool test_wait_has_read_only(int fd)
{
	struct drm_i915_gem_wait arg = { .flags = I915_WAIT_READ_ONLY };
	int err = 0;
	if (ioctl(fd, DRM_IOCTL_I915_GEM_WAIT, &arg))
		err = -errno;
	return err == -ENOENT;
}

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 5 +++--
 include/uapi/drm/i915_drm.h     | 1 +
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index c2a660c7538b..d50710752265 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3006,7 +3006,7 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	ktime_t start;
 	long ret;
 
-	if (args->flags != 0)
+	if (args->flags & ~(I915_WAIT_READ_ONLY))
 		return -EINVAL;
 
 	obj = i915_gem_object_lookup(file, args->bo_handle);
@@ -3016,7 +3016,8 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	start = ktime_get();
 
 	ret = i915_gem_object_wait(obj,
-				   I915_WAIT_INTERRUPTIBLE | I915_WAIT_ALL,
+				   I915_WAIT_INTERRUPTIBLE |
+				   (args->flags & I915_WAIT_READ_ONLY ? 0 : I915_WAIT_ALL),
 				   to_wait_timeout(args->timeout_ns),
 				   to_rps_client(file));
 
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 72460826f818..6387a7485702 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1312,6 +1312,7 @@ struct drm_i915_gem_wait {
 	/** Handle of BO we shall wait on */
 	__u32 bo_handle;
 	__u32 flags;
+#define I915_WAIT_READ_ONLY		0x1
 	/** Number of nanoseconds to wait, Returns time remaining. */
 	__s64 timeout_ns;
 };
-- 
2.11.0