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

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Date:   Wed, 29 Mar 2023 12:39:25 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     David Gow <davidgow@...gle.com>,
        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:     dri-devel@...ts.freedesktop.org, David Gow <davidgow@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm: test: Fix 32-bit issue in drm_buddy_test

On Wed, 29 Mar 2023, David Gow <davidgow@...gle.com> wrote:
> 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)) {

Then maybe use is_power_of_2_u64() instead?

BR,
Jani.

>  		kunit_err(test, "block size not power of two\n");
>  		err = -EINVAL;
>  	}

-- 
Jani Nikula, Intel Open Source Graphics Center