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Date:   Sun, 8 Mar 2020 17:13:15 +0100
From:   Oscar Carter <oscar.carter@gmx.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Forest Bond <forest@alittletooquiet.net>,
        devel@driverdev.osuosl.org, Malcolm Priestley <tvboxspy@gmail.com>,
        linux-kernel@vger.kernel.org,
        Gabriela Bittencourt <gabrielabittencourt00@gmail.com>,
        Colin Ian King <colin.king@canonical.com>,
        Oscar Carter <oscar.carter@gmx.com>
Subject: Re: [PATCH] staging: vt6656: Use BIT_ULL() macro instead of bit
 shift operation
Message-ID: <20200308161047.GA3285@ubuntu>
References: <20200307104929.7710-1-oscar.carter@gmx.com>
 <20200308065538.GF3983392@kroah.com>
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On Sun, Mar 08, 2020 at 07:55:38AM +0100, Greg Kroah-Hartman wrote:
> On Sat, Mar 07, 2020 at 11:49:29AM +0100, Oscar Carter wrote:
> > Replace the bit left shift operation with the BIT_ULL() macro and remo=
ve
> > the unnecessary "and" operation against the bit_nr variable.
> >
> > Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
> > ---
> >  drivers/staging/vt6656/main_usb.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/staging/vt6656/main_usb.c b/drivers/staging/vt665=
6/main_usb.c
> > index 5e48b3ddb94c..f7ca9e97594d 100644
> > --- a/drivers/staging/vt6656/main_usb.c
> > +++ b/drivers/staging/vt6656/main_usb.c
> > @@ -21,6 +21,7 @@
> >   */
> >  #undef __NO_VERSION__
> >
> > +#include <linux/bits.h>
> >  #include <linux/etherdevice.h>
> >  #include <linux/file.h>
> >  #include "device.h"
> > @@ -802,8 +803,7 @@ static u64 vnt_prepare_multicast(struct ieee80211_=
hw *hw,
> >
> >  	netdev_hw_addr_list_for_each(ha, mc_list) {
> >  		bit_nr =3D ether_crc(ETH_ALEN, ha->addr) >> 26;
> > -
> > -		mc_filter |=3D 1ULL << (bit_nr & 0x3f);
> > +		mc_filter |=3D BIT_ULL(bit_nr);
>
> Are you sure this does the same thing?  You are not masking off bit_nr
> anymore, why not?

My reasons are exposed below:

The ether_crc function returns an u32 type (unsigned of 32 bits). Then the=
 right
shift operand discards the 26 lsb bits (the bits shifted off the right sid=
e are
discarded). The 6 msb bits of the u32 returned by the ether_crc function a=
re
positioned in bit 5 to bit 0 of the variable bit_nr. Due to the right shif=
t
happens over an unsigned type, the 26 new bits added on the left side will=
 be 0.

In summary, after the right bit shift operation we obtain in the variable =
bit_nr
(unsigned of 32 bits) the value represented by the 6 msb bits of the value
returned by the ether_crc function. So, only the 6 lsb bits of the variabl=
e
bit_nr are important. The 26 msb bits of this variable are 0.

In this situation, the "and" operation with the mask 0x3f (mask of 6 lsb b=
its)
is unnecessary due to its purpose is to reset (set to 0 value) the 26 msb =
bits
that are yet 0.
>
> thanks,
>
> greg k-h

thanks,

Oscar