Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     David Gow <davidgow@google.com>,
        =?utf-8?Q?Lu=C3=ADs?= Mendes <luis.p.mendes@gmail.com>,
        Christian =?utf-8?Q?K=C3=B6nig?= <christian.koenig@amd.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        =?utf-8?Q?Ma?= =?utf-8?Q?=C3=ADra?= Canal 
        <mairacanal@riseup.net>, Arthur Grillo <arthurgrillo@riseup.net>
Cc:     dri-devel@lists.freedesktop.org, David Gow <davidgow@google.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] drm: test: Fix 32-bit issue in drm_buddy_test
In-Reply-To: <87lejf522a.fsf@intel.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20230329065532.2122295-1-davidgow@google.com>
 <20230329065532.2122295-2-davidgow@google.com> <87lejf522a.fsf@intel.com>
Date:   Wed, 29 Mar 2023 12:42:35 +0300
Message-ID: <87ilej51x0.fsf@intel.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, 29 Mar 2023, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Wed, 29 Mar 2023, David Gow <davidgow@google.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@google.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?

*sigh* And as soon as I wrote that I realized it's a local thing in
btrfs. Never mind for now...

...but in the long run would be nice to either fix is_power_of_2() for
u64 or add that u64 version...

BR,
Jani.

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

-- 
Jani Nikula, Intel Open Source Graphics Center