linux-kernel - Re: [PATCH v4] staging: rtl8723bs: Use % 4096u instead of & 0xfff

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Message-ID: <CADYq+faDs-A6MB=JvfVi8jWznr3bLbT9n5HajKLZbd34+Z7Dog@mail.gmail.com>
Date: Mon, 7 Apr 2025 14:35:26 +0100
From: Samuel Abraham <abrahamadekunle50@...il.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Greg KH <gregkh@...uxfoundation.org>, julia.lawall@...ia.fr, 
	outreachy@...ts.linux.dev, linux-staging@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, dan.carpenter@...aro.org, andy@...nel.org
Subject: Re: [PATCH v4] staging: rtl8723bs: Use % 4096u instead of & 0xfff

On Mon, Apr 7, 2025 at 1:21 PM David Laight
<david.laight.linux@...il.com> wrote:
>
> On Mon, 7 Apr 2025 08:53:30 +0200
> Greg KH <gregkh@...uxfoundation.org> wrote:
>
> > On Mon, Apr 07, 2025 at 08:36:35AM +0200, Greg KH wrote:
> > > On Mon, Apr 07, 2025 at 06:30:50AM +0000, Abraham Samuel Adekunle wrote:
> > > > The sequence number is constrained to a range of [0, 4095], which
> > > > is a total of 4096 values. The bitmask operation using `0xfff` is
> > > > used to perform this wrap-around. While this is functionally correct,
> > > > it obscures the intended semantic of a 4096-based wrap.
> > > >
> > > > Using a modulo operation with `4096u` makes the wrap-around logic
> > >
> > > <snip>
> > >
> > > > -                         psta->sta_xmitpriv.txseq_tid[pattrib->priority] &= 0xFFF;
> > > > +                         psta->sta_xmitpriv.txseq_tid[pattrib->priority] &= 4096u;
> > >
> > > I do not see a modulo operation here, only another & operation.
> > >
> > > >                           pattrib->seqnum = psta->sta_xmitpriv.txseq_tid[pattrib->priority];
> > > >
> > > >                           SetSeqNum(hdr, pattrib->seqnum);
> > > > @@ -963,11 +963,11 @@ s32 rtw_make_wlanhdr(struct adapter *padapter, u8 *hdr, struct pkt_attrib *pattr
> > > >                                   if (SN_LESS(pattrib->seqnum, tx_seq)) {
> > > >                                           pattrib->ampdu_en = false;/* AGG BK */
> > > >                                   } else if (SN_EQUAL(pattrib->seqnum, tx_seq)) {
> > > > -                                         psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (tx_seq+1)&0xfff;
> > > > +                                         psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (tx_seq+1)&4096u;
> > >
> > > This also looks odd, nothing is being "AND" here, it's an address value
> > > being set (and an odd one at that, but that's another issue...)
> >
> > Sorry, no, I was wrong, it is being & here, but not %.  My fault,
> > the lack of spaces here threw me.
>
> It is still wrong '& 0xfff' => '% 4096u'.
> But it is all rather pointless especially if you can't test it.
>
> Plausibly more useful would be to find ALL of the uses of 0xfff/4096 (I suspect
> there is an array lurking somewhere) and change them to use the same constant.
> But you need to be able to test the changes - or at least discover that
> they make absolutely no difference to the generated object code.

Yes, thank you for this, David.

I will compare the generated object files for both cases and compare
them to make sure there is no difference.

Then, to check for other cases in the codebase, a semantic patch will
be useful, so I will write one to search for
such cases and change them to use the constant.

Thank you very much

Adekunle