Re: [PATCH 2/2] drm: test: Fix 32-bit issue in drm_buddy_test

[Christian König]

Am 29.03.23 um 13:28 schrieb Jani Nikula:
> On Wed, 29 Mar 2023, Maíra Canal <mcanal@xxxxxxxxxx> 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@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> >
> > 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;
> > >    	}