[PATCH v2 2/3] drm_buddy: fix power-of-2 rounding errs

[David Gow <david@davidgow.net>]

From: Jim Cromie <jim.cromie@gmail.com>

The standard roundup_pow_of_two() and rounddown_pow_of_two() macros use
unsigned long internally, which on 32-bit architectures (like arm32) is
a 32-bit type.

drm_test_buddy_alloc_exceeds_max_order() uses these on a u64 value,
where they silently truncate the 10GB allocation, giving unexpected
success in DRM-CI.  (see below the snip).

Fix this by replacing the those macros with the safe 64-bit power-of-two
equivalents added in the previous patch.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/all/20260326185413.1205870-40-jim.cromie@gmail.com/
Signed-off-by: David Gow <david@davidgow.net>
---

This should actually be version 13 or 14, I think, as it's just a rebase
of v12 here, where it was part of a large series of fixups:
https://lore.kernel.org/all/20260326185413.1205870-40-jim.cromie@gmail.com/

There are probably some other places where this is not perfectly 32-bit safe,
but this is at least enough to fix the KUnit tests.

The major changes since that version are:
- Add the helper functions rounddown_pow_of_two_u64() and
  roundup_pow_of_two_u64() (see patch 1) instead of open-coding them
- Rebase now that the buddy allocator lives in drivers/gpu/buddy.c
  instead of drivers/gpu/drm/drm_buddy.c

This is still breaking the gpu_test_buddy_alloc_exceeds_max_order
KUnit test on 32-bit systems:
[09:01:26]     # gpu_test_buddy_alloc_exceeds_max_order: EXPECTATION FAILED at drivers/gpu/tests/gpu_buddy_test.c:1429
[09:01:26]     Expected err == -22, but
[09:01:26]         err == 0 (0x0)
[09:01:26] WARNING: drivers/gpu/buddy.c:508 at gpu_buddy_fini+0x244/0x2e0, CPU#0: kunit_try_catch/1595
[09:01:26]     # gpu_test_buddy_alloc_exceeds_max_order: drivers/gpu/buddy.c:508: gpu_buddy_assert(gpu_buddy_block_is_free(mm->roots[i]))
[09:01:26] WARNING: drivers/gpu/buddy.c:516 at gpu_buddy_fini+0x284/0x2e0, CPU#0: kunit_try_catch/1595
[09:01:26]     # gpu_test_buddy_alloc_exceeds_max_order: drivers/gpu/buddy.c:508: gpu_buddy_assert(gpu_buddy_block_is_free(mm->roots[i]))
[09:01:26] WARNING: drivers/gpu/buddy.c:516 at gpu_buddy_fini+0x284/0x2e0, CPU#0: kunit_try_catch/1595
[09:01:26]     # gpu_test_buddy_alloc_exceeds_max_order: drivers/gpu/buddy.c:519: gpu_buddy_assert(!mm->used_scoreboard[i])
[09:01:26] [FAILED] gpu_test_buddy_alloc_exceeds_max_order

Changes since v1:
https://lore.kernel.org/all/20260821091918.1902032-2-david@ingeniumdigital.com/
- Rename round{up,down}_pow_of_two64() -> round{up,down_pow_of_two_u64()
  (This seems nicer and more consistent with what everyone else is doing)

---
 drivers/gpu/buddy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/buddy.c b/drivers/gpu/buddy.c
index a5553fcec28c..0c432719f8f1 100644
--- a/drivers/gpu/buddy.c
+++ b/drivers/gpu/buddy.c
@@ -1193,7 +1193,7 @@ static int __alloc_contig_try_harder(struct gpu_buddy *mm,
 	u64 modify_size;
 	int err;
 
-	modify_size = rounddown_pow_of_two(size);
+	modify_size = rounddown_pow_of_two_u64(size);
 	order = ilog2(modify_size) - ilog2(mm->chunk_size);
 	if (order == 0)
 		return -ENOSPC;
@@ -1440,7 +1440,7 @@ int gpu_buddy_alloc_blocks(struct gpu_buddy *mm,
 
 	/* Roundup the size to power of 2 */
 	if (flags & GPU_BUDDY_CONTIGUOUS_ALLOCATION) {
-		size = roundup_pow_of_two(size);
+		size = roundup_pow_of_two_u64(size);
 		min_block_size = size;
 		/*
 		 * Normalize the requested size to min_block_size for regular allocations.
-- 
2.55.0