So, while tinkering with my Dove DRM driver, I tripped over a BUG_ON() in drm_mm_hole_node_start(), which was called from drm_mm_dump_table():
int drm_mm_dump_table(struct seq_file *m, struct drm_mm *mm) { struct drm_mm_node *entry; unsigned long total_used = 0, total_free = 0, total = 0; unsigned long hole_start, hole_end, hole_size;
hole_start = drm_mm_hole_node_start(&mm->head_node); hole_end = drm_mm_hole_node_end(&mm->head_node);
drm_mm_hole_node_start() does this:
static inline unsigned long drm_mm_hole_node_start(struct drm_mm_node *hole_node) { BUG_ON(!hole_node->hole_follows); return __drm_mm_hole_node_start(hole_node); }
This appears to indicate that it is required that the head node should always have a "hole" following it.
As this used to work, I dug in the git history, and found commit 6b9d89b4 (drm: Add colouring to the range allocator), in particular this change:
static void drm_mm_insert_helper(struct drm_mm_node *hole_node, struct drm_mm_node *node, - unsigned long size, unsigned alignment) + unsigned long size, unsigned alignment, + unsigned long color) { ... - if (!tmp) { + if (alignment) { + unsigned tmp = adj_start % alignment; + if (tmp) + adj_start += alignment - tmp; + } + + if (adj_start == hole_start) { hole_node->hole_follows = 0; - list_del_init(&hole_node->hole_stack); - } else - wasted = alignment - tmp; + list_del(&hole_node->hole_stack); + }
Now, when we do an allocation, it is typically done as follows:
free = drm_mm_search_free(mm, size, align, 0); if (free) linear = drm_mm_get_block(free, size, align);
If nothing has been allocated in the mm, 'free' will be &mm->head_node. The result is that drm_mm_get_block calls down into drm_mm_insert_helper with hole_node == &mm->head_node. This I confirmed via:
diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c index db1e2d6..7241be8 100644 --- a/drivers/gpu/drm/drm_mm.c +++ b/drivers/gpu/drm/drm_mm.c @@ -125,6 +125,7 @@ static void drm_mm_insert_helper(struct drm_mm_node *hole_node, }
if (adj_start == hole_start) { + BUG_ON(hole_node == &mm->head_node); hole_node->hole_follows = 0; list_del(&hole_node->hole_stack); }
which triggers during booting. Hence, we end up setting the head_node to indicate no hole following - which violates the conditions of other parts of the code in drm_mm.c.
It looks like this change was made quite a while ago - back in July 2012, so I assume that while the rest of the allocator is fine with this, it's just the debugging code which is now broken. However, I suspect that DRM people may wish to either back out the coloring change or carefully review the change for correctness throughout the drm_mm code.