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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=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 1/2] drm: buddy_allocator: Fix buddy allocator init on
 32-bit systems
In-Reply-To: <20230329065532.2122295-1-davidgow@google.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20230329065532.2122295-1-davidgow@google.com>
Date:   Thu, 30 Mar 2023 13:53:47 +0300
Message-ID: <878rfe4iis.fsf@intel.com>
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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 p=
atch
> 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-CviDQbNaH0qfgYT=
SSjZgvvyj4U78AA@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, u6=
4 chunk_size)
>  		unsigned int order;
>  		u64 root_size;
>=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.


BR,
Jani.


>=20=20
>  		root =3D drm_block_alloc(mm, NULL, order, offset);
>  		if (!root)

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