linux-kernel - RE: [PATCH 10/54] net: ethernet: replace bitmap_weight with bitmap

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Date:   Tue, 25 Jan 2022 22:14:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Yury Norov' <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joe Perches <joe@...ches.com>,
        "Dennis Zhou" <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        "Nicholas Piggin" <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ariel Elior <aelior@...vell.com>,
        Manish Chopra <manishc@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 10/54] net: ethernet: replace bitmap_weight with
 bitmap_empty for qlogic

From: Yury Norov
> Sent: 25 January 2022 21:10
> On Mon, Jan 24, 2022 at 4:29 AM Andy Shevchenko
> <andriy.shevchenko@...ux.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.
You just can't cast the argument it has to be long[].

	David

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