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

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Date:   Wed, 29 Mar 2023 15:14:56 +0200
From:   Christian König <christian.koenig@....com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Maíra Canal <mcanal@...lia.com>,
        David Gow <davidgow@...gle.com>,
        Luís Mendes <luis.p.mendes@...il.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:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] drm: test: Fix 32-bit issue in drm_buddy_test

Am 29.03.23 um 13:28 schrieb Jani Nikula:
> On Wed, 29 Mar 2023, Maíra Canal <mcanal@...lia.com> wrote:
>> On 3/29/23 03:55, David Gow 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>
>> As we still didn't consolidate an implementation of is_power_of_2_u64(),
> I just cooked up some patches to try to make is_power_of_2() more
> flexible. I only sent them to the "CI trybot" for a quick spin first,
> will post to lkml later. [1]

In the meantime I'm pushing this to drm-misc-fixes unless somebody has 
some last second objections.

Christian.

>
> BR,
> Jani.
>
>
> [1] https://patchwork.freedesktop.org/series/115785/
>
>> Reviewed-by: Maíra Canal <mcanal@...lia.com>
>>
>> Best Regards,
>> - Maíra Canal
>>
>>> ---
>>>
>>> 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;
>>>    	}