[Intel-gfx] [PATCH 3/6] drm/i915: Support write only mappings

[Ben Widawsky]

When doing a GTT mapping, the pages are not backed until taking a fault.
If we know that the object is write only, when the fault happens we do not need
to make the pages coherent with the CPU.  This allows for semi fast prefaults
to occur in libdrm at map time.

For the non-GTT case, there isn't much to do except make sure we handle things
properly we we move an object out of the GTT.

Finally, modify the set domain IOCTL to actually mark objects as write only.
Current limitation in that once an object is marked write only, it cannot be
readable.

Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem.c |   32 ++++++++++++++++++++++++--------
 1 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d4de7f7..48ea4bc 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1047,6 +1047,7 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
 	struct drm_i915_gem_object *obj;
 	uint32_t read_domains = args->read_domains;
 	uint32_t write_domain = args->write_domain;
+	int write_only = 0;
 	int ret;
 
 	if (!(dev->driver->driver_features & DRIVER_GEM))
@@ -1059,11 +1060,15 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
 	if (read_domains & I915_GEM_GPU_DOMAINS)
 		return -EINVAL;
 
-	/* Having something in the write domain implies it's in the read
-	 * domain, and only that read domain.  Enforce that in the request.
-	 */
-	if (write_domain != 0 && read_domains != write_domain)
+	if (write_domain != 0 && read_domains == 0) {
+		write_only = 1;
+	} else if (write_domain != 0 && read_domains != write_domain) {
+		/* Having something in the write domain implies it's in the read
+		 * domain, and only that read domain. Enforce that in the
+		 * request.
+		 */
 		return -EINVAL;
+	}
 
 	ret = i915_mutex_lock_interruptible(dev);
 	if (ret)
@@ -1084,6 +1089,13 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
 		 */
 		if (ret == -EINVAL)
 			ret = 0;
+	} else if (obj->cpu_write_only && !write_only) {
+		DRM_ERROR("Not yet supported\n");
+		ret = -EINVAL;
+		goto unlock;
+	} else if (write_only) {
+		atomic_set(&obj->cpu_write_only, 1);
+		obj->base.read_domains = 0;
 	} else {
 		ret = i915_gem_object_set_to_cpu_domain(obj, write_domain != 0);
 	}
@@ -1217,9 +1229,13 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 		if (ret)
 			goto unlock;
 
-		ret = i915_gem_object_set_to_gtt_domain(obj, write);
-		if (ret)
-			goto unlock;
+		if (obj->cpu_write_only) {
+			i915_gem_object_flush_cpu_write_domain(obj);
+		} else {
+			ret = i915_gem_object_set_to_gtt_domain(obj, write);
+			if (ret)
+				goto unlock;
+		}
 	}
 
 	if (obj->tiling_mode == I915_TILING_NONE)
@@ -1586,7 +1602,7 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj)
 		obj->dirty = 0;
 
 	for (i = 0; i < page_count; i++) {
-		if (obj->dirty)
+		if (obj->dirty || obj->cpu_write_only)
 			set_page_dirty(obj->pages[i]);
 
 		if (obj->madv == I915_MADV_WILLNEED)
-- 
1.7.6.1