linux-kernel - [PATCH 1/2] drm: buddy_allocator: Fix buddy allocator init on 32-bit systems

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Date:   Wed, 29 Mar 2023 14:55:32 +0800
From:   David Gow <davidgow@...gle.com>
To:     "Luís Mendes" <luis.p.mendes@...il.com>,
        "Christian König" <christian.koenig@....com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        "Maíra Canal" <mairacanal@...eup.net>,
        Arthur Grillo <arthurgrillo@...eup.net>
Cc:     David Gow <davidgow@...gle.com>, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] drm: buddy_allocator: Fix buddy allocator init on 32-bit systems

The drm buddy allocator tests were broken on 32-bit systems, as
rounddown_pow_of_two() takes a long, and the buddy allocator handles
64-bit sizes even on 32-bit systems.

This can be reproduced with the drm_buddy_allocator KUnit tests on i386:
	./tools/testing/kunit/kunit.py run --arch i386 \
	--kunitconfig ./drivers/gpu/drm/tests drm_buddy

(It results in kernel BUG_ON() when too many blocks are created, due to
the block size being too small.)

This was independently uncovered (and fixed) by Luís Mendes, whose patch
added a new u64 variant of rounddown_pow_of_two(). This version instead
recalculates the size based on the order.

Reported-by: Luís Mendes <luis.p.mendes@...il.com>
Link: https://lore.kernel.org/lkml/CAEzXK1oghXAB_KpKpm=-CviDQbNaH0qfgYTSSjZgvvyj4U78AA@mail.gmail.com/T/
Signed-off-by: David Gow <davidgow@...gle.com>
---
 drivers/gpu/drm/drm_buddy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
index 3d1f50f481cf..7098f125b54a 100644
--- a/drivers/gpu/drm/drm_buddy.c
+++ b/drivers/gpu/drm/drm_buddy.c
@@ -146,8 +146,8 @@ int drm_buddy_init(struct drm_buddy *mm, u64 size, u64 chunk_size)
 		unsigned int order;
 		u64 root_size;
 
-		root_size = rounddown_pow_of_two(size);
-		order = ilog2(root_size) - ilog2(chunk_size);
+		order = ilog2(size) - ilog2(chunk_size);
+		root_size = chunk_size << order;
 
 		root = drm_block_alloc(mm, NULL, order, offset);
 		if (!root)
-- 
2.40.0.348.gf938b09366-goog