On 13/01/2017 10:33, Chris Wilson wrote:
Reading the ggtt_views is much more pleasant without the extra
characters from specifying the union (i.e. ggtt_view.partial rather than
ggtt_view.params.partial). To make this work inside i915_vma_compare()
with only a single memcmp requires us to ensure that there are no
uninitialised bytes within each branch of the union (we make sure the
structs are packed) and we need to store the size of each branch.

v4: Rewrite changelog and add comments explaining the assert.

Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
---
 drivers/gpu/drm/i915/i915_debugfs.c  | 20 ++++++++++----------
 drivers/gpu/drm/i915/i915_gem.c      |  8 ++++----
 drivers/gpu/drm/i915/i915_gem_gtt.c  |  9 ++++-----
 drivers/gpu/drm/i915/i915_gem_gtt.h  |  2 +-
 drivers/gpu/drm/i915/i915_vma.c      |  9 ++++-----
 drivers/gpu/drm/i915/i915_vma.h      |  9 ++++++++-
 drivers/gpu/drm/i915/intel_display.c |  2 +-
 7 files changed, 32 insertions(+), 27 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c 
b/drivers/gpu/drm/i915/i915_debugfs.c
index e367f06f5883..da13c4c3aa6b 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -167,20 +167,20 @@ describe_obj(struct seq_file *m, struct 
drm_i915_gem_object *obj)

                        case I915_GGTT_VIEW_PARTIAL:
                                seq_printf(m, ", partial [%08llx+%x]",
-                                          vma->ggtt_view.params.partial.offset 
<< PAGE_SHIFT,
-                                          vma->ggtt_view.params.partial.size 
<< PAGE_SHIFT);
+                                          vma->ggtt_view.partial.offset << 
PAGE_SHIFT,
+                                          vma->ggtt_view.partial.size << 
PAGE_SHIFT);
                                break;

                        case I915_GGTT_VIEW_ROTATED:
                                seq_printf(m, ", rotated [(%ux%u, stride=%u, 
offset=%u), (%ux%u, stride=%u, offset=%u)]",
-                                          
vma->ggtt_view.params.rotated.plane[0].width,
-                                          
vma->ggtt_view.params.rotated.plane[0].height,
-                                          
vma->ggtt_view.params.rotated.plane[0].stride,
-                                          
vma->ggtt_view.params.rotated.plane[0].offset,
-                                          
vma->ggtt_view.params.rotated.plane[1].width,
-                                          
vma->ggtt_view.params.rotated.plane[1].height,
-                                          
vma->ggtt_view.params.rotated.plane[1].stride,
-                                          
vma->ggtt_view.params.rotated.plane[1].offset);
+                                          
vma->ggtt_view.rotated.plane[0].width,
+                                          
vma->ggtt_view.rotated.plane[0].height,
+                                          
vma->ggtt_view.rotated.plane[0].stride,
+                                          
vma->ggtt_view.rotated.plane[0].offset,
+                                          
vma->ggtt_view.rotated.plane[1].width,
+                                          
vma->ggtt_view.rotated.plane[1].height,
+                                          
vma->ggtt_view.rotated.plane[1].stride,
+                                          
vma->ggtt_view.rotated.plane[1].offset);
                                break;

                        default:
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index f034d8d2dd4c..d8622fd23f5d 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1760,10 +1760,10 @@ compute_partial_view(struct drm_i915_gem_object *obj,
                chunk = roundup(chunk, tile_row_pages(obj));

        view.type = I915_GGTT_VIEW_PARTIAL;
-       view.params.partial.offset = rounddown(page_offset, chunk);
-       view.params.partial.size =
+       view.partial.offset = rounddown(page_offset, chunk);
+       view.partial.size =
                min_t(unsigned int, chunk,
-                     (obj->base.size >> PAGE_SHIFT) - 
view.params.partial.offset);
+                     (obj->base.size >> PAGE_SHIFT) - view.partial.offset);

        /* If the partial covers the entire object, just create a normal VMA. */
        if (chunk >= obj->base.size >> PAGE_SHIFT)
@@ -1879,7 +1879,7 @@ int i915_gem_fault(struct vm_area_struct *area, struct 
vm_fault *vmf)

        /* Finally, remap it using the new GTT offset */
        ret = remap_io_mapping(area,
-                              area->vm_start + (vma->ggtt_view.params.partial.offset 
<< PAGE_SHIFT),
+                              area->vm_start + (vma->ggtt_view.partial.offset 
<< PAGE_SHIFT),
                               (ggtt->mappable_base + vma->node.start) >> 
PAGE_SHIFT,
                               min_t(u64, vma->size, area->vm_end - 
area->vm_start),
                               &ggtt->mappable);
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c 
b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 6d2ff20ec973..06cfd6951a5e 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -3490,7 +3490,7 @@ intel_partial_pages(const struct i915_ggtt_view *view,
 {
        struct sg_table *st;
        struct scatterlist *sg, *iter;
-       unsigned int count = view->params.partial.size;
+       unsigned int count = view->partial.size;
        unsigned int offset;
        int ret = -ENOMEM;

@@ -3502,9 +3502,7 @@ intel_partial_pages(const struct i915_ggtt_view *view,
        if (ret)
                goto err_sg_alloc;

-       iter = i915_gem_object_get_sg(obj,
-                                     view->params.partial.offset,
-                                     &offset);
+       iter = i915_gem_object_get_sg(obj, view->partial.offset, &offset);
        GEM_BUG_ON(!iter);

        sg = st->sgl;
@@ -3556,7 +3554,8 @@ i915_get_ggtt_vma_pages(struct i915_vma *vma)
                vma->pages = vma->obj->mm.pages;
        else if (vma->ggtt_view.type == I915_GGTT_VIEW_ROTATED)
                vma->pages =
-                       intel_rotate_fb_obj_pages(&vma->ggtt_view.params.rotated, 
vma->obj);
+                       intel_rotate_fb_obj_pages(&vma->ggtt_view.rotated,
+                                                 vma->obj);
        else if (vma->ggtt_view.type == I915_GGTT_VIEW_PARTIAL)
                vma->pages = intel_partial_pages(&vma->ggtt_view, vma->obj);
        else
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h 
b/drivers/gpu/drm/i915/i915_gem_gtt.h
index 20c03c30ce4e..560fd8ddaf2c 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.h
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
@@ -179,7 +179,7 @@ struct i915_ggtt_view {
                /* Members need to contain no holes/padding */
                struct intel_partial_info partial;
                struct intel_rotation_info rotated;
-       } params;
+       };
 };

 extern const struct i915_ggtt_view i915_ggtt_view_normal;
diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index b74eeb73ae41..e6c339b0ea37 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -97,15 +97,14 @@ __i915_vma_create(struct drm_i915_gem_object *obj,
                vma->ggtt_view = *view;
                if (view->type == I915_GGTT_VIEW_PARTIAL) {
                        GEM_BUG_ON(range_overflows_t(u64,
-                                                    
view->params.partial.offset,
-                                                    view->params.partial.size,
+                                                    view->partial.offset,
+                                                    view->partial.size,
                                                     obj->base.size >> 
PAGE_SHIFT));
-                       vma->size = view->params.partial.size;
+                       vma->size = view->partial.size;
                        vma->size <<= PAGE_SHIFT;
                        GEM_BUG_ON(vma->size >= obj->base.size);
                } else if (view->type == I915_GGTT_VIEW_ROTATED) {
-                       vma->size =
-                               intel_rotation_info_size(&view->params.rotated);
+                       vma->size = intel_rotation_info_size(&view->rotated);
                        vma->size <<= PAGE_SHIFT;
                }
        }
diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
index c803afa2d619..c4a2294ee42d 100644
--- a/drivers/gpu/drm/i915/i915_vma.h
+++ b/drivers/gpu/drm/i915/i915_vma.h
@@ -209,13 +209,20 @@ i915_vma_compare(struct i915_vma *vma,
                return cmp;

        BUILD_BUG_ON(I915_GGTT_VIEW_PARTIAL == I915_GGTT_VIEW_ROTATED);
+       BUILD_BUG_ON(offsetof(typeof(*view), rotated) !=
+                    offsetof(typeof(*view), partial));

        /* ggtt_view.type also encodes its size so that we both distinguish
         * different views using it as a "type" and also use a compact (no
         * accessing of uninitialised padding bytes) memcmp without storing
         * an extra parameter or adding more code.
+        *
+        * To ensure that the memcmp is valid for all branches of the union,
+        * even though the code looks like it is just comparing one branch,
+        * we assert above that all branches have the same address, and that
+        * each branch has a unique type/size.
         */
-       return memcmp(&vma->ggtt_view.params, &view->params, view->type);
+       return memcmp(&vma->ggtt_view.partial, &view->partial, view->type);
 }

 int i915_vma_bind(struct i915_vma *vma, enum i915_cache_level cache_level,
diff --git a/drivers/gpu/drm/i915/intel_display.c 
b/drivers/gpu/drm/i915/intel_display.c
index fd5fbc83c69e..f4be20f0198a 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2139,7 +2139,7 @@ intel_fill_fb_ggtt_view(struct i915_ggtt_view *view,
 {
        if (drm_rotation_90_or_270(rotation)) {
                *view = i915_ggtt_view_rotated;
-               view->params.rotated = to_intel_framebuffer(fb)->rot_info;
+               view->rotated = to_intel_framebuffer(fb)->rot_info;
        } else {
                *view = i915_ggtt_view_normal;
        }


Looks ok. But please ask for some second opinions on the series.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursu...@intel.com>

In the meantime I came up with a softer "revert" idea, if it comes to play, which could be adding a params size field to the view structure.

Regards,

Tvrtko
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