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

[Matthew Auld]

On 21/08/2026 10:19, David Gow wrote:
> From: Jim Cromie <jim.cromie@xxxxxxxxx>
>
> 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.

Do we also need something like is_power_of_2_u64() ? I think we could 
then remove that from i915_utils.h

> Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
> Signed-off-by: David Gow <david@xxxxxxxxxxxx>
> ---
>
> This should actually be version 13, 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@xxxxxxxxx/
>
> The major changes since that version are:
> - Add the helper functions rounddown_pow_of_two64() and
>    roundup_pow_of_two64() (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
>
> Cheers,
> -- David
>
> ---
>   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..64f41f1fe908 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_two64(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_two64(size);
>   		min_block_size = size;
>   		/*
>   		 * Normalize the requested size to min_block_size for regular allocations.