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

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Message-ID: <Z_QV07zc5B2Xc2_L@smile.fi.intel.com>
Date: Mon, 7 Apr 2025 21:13:39 +0300
From: Andy Shevchenko <andy@...nel.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Greg KH <gregkh@...uxfoundation.org>,
	Abraham Samuel Adekunle <abrahamadekunle50@...il.com>,
	julia.lawall@...ia.fr, outreachy@...ts.linux.dev,
	linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
	dan.carpenter@...aro.org
Subject: Re: [PATCH v4] staging: rtl8723bs: Use % 4096u instead of & 0xfff

On Mon, Apr 07, 2025 at 07:06:45PM +0100, David Laight wrote:
> On Mon, 7 Apr 2025 15:28:34 +0300
> Andy Shevchenko <andy@...nel.org> wrote:
> > On Mon, Apr 07, 2025 at 01:21:15PM +0100, David Laight 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:    

<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'.  
> > 
> > Why?
> 
> Do some math :-)

Can you be more specific, please?

> > > 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.  
> > 
> > The problem with &, it's not non-power-of-2 tolerable solution. Also using
> > hexadecimal there is not so helpful as when we are talking about sequences
> > (or indices in the circular buffer), the decimal makes more sense.
> 
> Except that you either want your circular buffer size to be a power of 2
> or reduce with a conditional (eg: if (++x == SIZE) x = 0;) not a divide.

Where do you see a divide in the generated code? And if even so, it means that
compiler optimiser thinks that it's not worse than the bit operations.

-- 
With Best Regards,
Andy Shevchenko