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Date:   Sat, 14 Mar 2020 16:06:13 +0100
From:   Oscar Carter <oscar.carter@gmx.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Malcolm Priestley <tvboxspy@gmail.com>, devel@driverdev.osuosl.org,
        Colin Ian King <colin.king@canonical.com>,
        Forest Bond <forest@alittletooquiet.net>,
        Gabriela Bittencourt <gabrielabittencourt00@gmail.com>,
        linux-kernel@vger.kernel.org, Oscar Carter <oscar.carter@gmx.com>
Subject: Re: [PATCH] staging: vt6656: Use BIT_ULL() macro instead of bit
 shift operation
Message-ID: <20200314150612.GA3153@ubuntu>
References: <20200307104929.7710-1-oscar.carter@gmx.com>
 <20200308065538.GF3983392@kroah.com>
 <20200308161047.GA3285@ubuntu>
 <561bc968-f88c-40e3-f53c-5c03f74f75ea@gmail.com>
 <20200310095011.GC2516963@kroah.com>
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On Tue, Mar 10, 2020 at 10:50:11AM +0100, Greg Kroah-Hartman wrote:
> On Sun, Mar 08, 2020 at 07:22:07PM +0000, Malcolm Priestley wrote:
> > >>>   */
> > >>>  #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 ieee80=
211_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). Th=
en the right
> > > shift operand discards the 26 lsb bits (the bits shifted off the rig=
ht side are
> > > discarded). The 6 msb bits of the u32 returned by the ether_crc func=
tion are
> > > positioned in bit 5 to bit 0 of the variable bit_nr. Due to the righ=
t shift
> > > happens over an unsigned type, the 26 new bits added on the left sid=
e will be 0.
> > >
> > > In summary, after the right bit shift operation we obtain in the var=
iable 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 v=
ariable
> > > 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 bits)
> > > is unnecessary due to its purpose is to reset (set to 0 value) the 2=
6 msb bits
> > > that are yet 0.
> >
> > The mask is only there out of legacy originally it was 31(0x1f) and th=
e
> > bit_nr spread across two mc_filter u32 arrays.
> >
> > The mask is not needed now it is u64.
> >
> > The patch is fine.
>
> Ok, then the changelog needs to be fixed up to explain all of this and
> resent.

Ok, I will create a new version patch with all of this information and I w=
ill
resend it.

>
> thanks,
>
> greg k-h

thanks,

Oscar