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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Christian =?utf-8?Q?K=C3=B6nig?= <christian.koenig@amd.com>,
        David Gow <davidgow@google.com>,
        =?utf-8?Q?Lu=C3=ADs?= Mendes <luis.p.mendes@gmail.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, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] drm: buddy_allocator: Fix buddy allocator init on
 32-bit systems
In-Reply-To: <e1af591e-a183-9b22-f533-297934e15e30@amd.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20230329065532.2122295-1-davidgow@google.com>
 <878rfe4iis.fsf@intel.com> <e1af591e-a183-9b22-f533-297934e15e30@amd.com>
Date:   Thu, 30 Mar 2023 14:12:17 +0300
Message-ID: <875yai4hny.fsf@intel.com>
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On Thu, 30 Mar 2023, Christian K=C3=B6nig <christian.koenig@amd.com> wrote:
> Am 30.03.23 um 12:53 schrieb Jani Nikula:
>> On Wed, 29 Mar 2023, David Gow <davidgow@google.com> wrote:
>>> 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 Lu=C3=ADs 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: Lu=C3=ADs Mendes <luis.p.mendes@gmail.com>
>>> Link: https://lore.kernel.org/lkml/CAEzXK1oghXAB_KpKpm=3D-CviDQbNaH0qfg=
YTSSjZgvvyj4U78AA@mail.gmail.com/T/
>>> Signed-off-by: David Gow <davidgow@google.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;
>>>=20=20=20
>>> -		root_size =3D rounddown_pow_of_two(size);
>>> -		order =3D ilog2(root_size) - ilog2(chunk_size);
>>> +		order =3D ilog2(size) - ilog2(chunk_size);
>>> +		root_size =3D chunk_size << order;
>> Just noticed near the beginning of the function there's also:
>>
>> 	if (!is_power_of_2(chunk_size))
>> 		return -EINVAL;
>>
>> which is also wrong for 32-bit.
>
> Yeah, but that isn't vital. We just use u64 for the chunk_size for=20
> consistency.
>
> In reality I wouldn't except more than 256K here.

Right. It's just not pedantically correct either. ;)

is_power_of_2() is pretty scary as it is, since it just truncates.

BR,
Jani.


>
> Regards,
> Christian.
>
>>
>>
>> BR,
>> Jani.
>>
>>
>>>=20=20=20
>>>   		root =3D drm_block_alloc(mm, NULL, order, offset);
>>>   		if (!root)
>

--=20
Jani Nikula, Intel Open Source Graphics Center