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SubjectRe: [PATCH 10/54] net: ethernet: replace bitmap_weight with bitmap_empty for qlogic
On Tue, Jan 25, 2022 at 2:15 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Yury Norov
> > Sent: 25 January 2022 21:10
> > On Mon, Jan 24, 2022 at 4:29 AM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
> > >
> > > On Sun, Jan 23, 2022 at 10:38:41AM -0800, Yury Norov wrote:
> > > > qlogic/qed code calls bitmap_weight() to check if any bit of a given
> > > > bitmap is set. It's better to use bitmap_empty() in that case because
> > > > bitmap_empty() stops traversing the bitmap as soon as it finds first
> > > > set bit, while bitmap_weight() counts all bits unconditionally.
> > >
> > > > - if (bitmap_weight((unsigned long *)&pmap[item], 64 * 8))
> > > > + if (!bitmap_empty((unsigned long *)&pmap[item], 64 * 8))
> > >
> > > > - (bitmap_weight((unsigned long *)&pmap[item],
> > > > + (!bitmap_empty((unsigned long *)&pmap[item],
> > >
> > > Side note, these castings reminds me previous discussion and I'm wondering
> > > if you have this kind of potentially problematic places in your TODO as
> > > subject to fix.
> >
> > In the discussion you mentioned above, the u32* was cast to u64*,
> > which is wrong. The code
> > here is safe because in the worst case, it casts u64* to u32*. This
> > would be OK wrt
> > -Werror=array-bounds.
> >
> > The function itself looks like doing this unsigned long <-> u64
> > conversions just for printing
> > purpose. I'm not a qlogic expert, so let's wait what people say?
>
> It'll be wrong on BE systems.

The bitmap_weigh() result will be correct. As you can see, the address
is 64-bit aligned anyways. The array boundary violation will never happen
as well.

DP_NOTICE() may be wrong, or may not. It depends on how important
the absolute position of the bit in the printed bitmap is. Nevertheless,
printk("%pb") is better and should be used.

This whole concern may be simply irrelevant if QED is not supported
on 32-bit BE machines. From what I can see, at least Infiniband requires
64BIT.

Thanks,
Yury

> You just can't cast the argument it has to be long[].
>
> David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

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