linux-kernel - [PATCH 2/2] drm: test: Fix 32-bit issue in drm_buddy

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Date:   Wed, 29 Mar 2023 14:55:34 +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 2/2] drm: test: Fix 32-bit issue in drm_buddy_test

The drm_buddy_test KUnit tests verify that returned blocks have sizes
which are powers of two using is_power_of_2(). However, is_power_of_2()
operations on a 'long', but the block size is a u64. So on systems where
long is 32-bit, this can sometimes fail even on correctly sized blocks.

This only reproduces randomly, as the parameters passed to the buddy
allocator in this test are random. The seed 0xb2e06022 reproduced it
fine here.

For now, just hardcode an is_power_of_2() implementation using
x & (x - 1).

Signed-off-by: David Gow <davidgow@...gle.com>
---

There are actually a couple of is_power_of_2_u64() implementations
already around in:
- drivers/gpu/drm/i915/i915_utils.h
- fs/btrfs/misc.h (called is_power_of_two_u64) 

So the ideal thing would be to consolidate these in one place.


---
 drivers/gpu/drm/tests/drm_buddy_test.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/tests/drm_buddy_test.c b/drivers/gpu/drm/tests/drm_buddy_test.c
index f8ee714df396..09ee6f6af896 100644
--- a/drivers/gpu/drm/tests/drm_buddy_test.c
+++ b/drivers/gpu/drm/tests/drm_buddy_test.c
@@ -89,7 +89,8 @@ static int check_block(struct kunit *test, struct drm_buddy *mm,
 		err = -EINVAL;
 	}
 
-	if (!is_power_of_2(block_size)) {
+	/* We can't use is_power_of_2() for a u64 on 32-bit systems. */
+	if (block_size & (block_size - 1)) {
 		kunit_err(test, "block size not power of two\n");
 		err = -EINVAL;
 	}
-- 
2.40.0.348.gf938b09366-goog