[PATCH v5] staging: rtl8723bs: Use % 4096u instead of & 0xfff

[PATCH v5] staging: rtl8723bs: Use % 4096u instead of & 0xfff

[Abraham Samuel Adekunle]

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 `% 4096u` makes the wrap-around logic
explicit and easier to understand. It clearly signals that the
sequence number cycles through a range of 4096 values.
It also makes the code robust against potential changes of the 4096
upper limit, especially when it becomes a non power of 2 value while
the AND(&) works solely for power of 2 values.

The use of `% 4096u` also guarantees that the modulo operation is
performed with unsigned arithmetic, preventing potential issues with
the signed types.

Suggested-by: Andy Shevchenko <andy@kernel.org>
Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Abraham Samuel Adekunle <abrahamadekunle50@gmail.com>
---
Changes in v4:
	- Corrected patch to use '%' instead of '&'
	- To ensure this change does not affect the functional
	behaviour, I compared the generated object files before and
	after the change using the `cmp` which compares the two
	object files byte by byte as shown below:

	$ make drivers/staging/rtl8723bs/core/rtw_xmit.o
	$ cmp rtw_xmit_before.o rtw_xmit_after.o

	No differences were found in the output, confirming that the
	change does not alter the compiled output.
Changes in v3:
	- Added more description to the commit message.
	- Removed blank line in the tag block.
	- Added more patch recipients.
Changes in v2:
	- Changed the commit message t a more descriptive message which
	makes it clear why the patch does the change.
	- changed the subject title to include `4096u` to show that an
	unsigned module is used.
Changes in v1:
	- Added more patch recipients.

 drivers/staging/rtl8723bs/core/rtw_xmit.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/rtl8723bs/core/rtw_xmit.c b/drivers/staging/rtl8723bs/core/rtw_xmit.c
index 026061b464f7..548fcf9968bd 100644
--- a/drivers/staging/rtl8723bs/core/rtw_xmit.c
+++ b/drivers/staging/rtl8723bs/core/rtw_xmit.c
@@ -943,7 +943,7 @@ s32 rtw_make_wlanhdr(struct adapter *padapter, u8 *hdr, struct pkt_attrib *pattr
 
 			if (psta) {
 				psta->sta_xmitpriv.txseq_tid[pattrib->priority]++;
-				psta->sta_xmitpriv.txseq_tid[pattrib->priority] &= 0xFFF;
+				psta->sta_xmitpriv.txseq_tid[pattrib->priority] %= 4096u;
 				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;
 
 						pattrib->ampdu_en = true;/* AGG EN */
 					} else {
-						psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (pattrib->seqnum+1)&0xfff;
+						psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (pattrib->seqnum+1)%4096u;
 						pattrib->ampdu_en = true;/* AGG EN */
 					}
 				}
-- 
2.34.1

Re: [PATCH v5] staging: rtl8723bs: Use % 4096u instead of & 0xfff

[Andy Shevchenko]

On Mon, Apr 7, 2025 at 6:54 PM Abraham Samuel Adekunle
<abrahamadekunle50@gmail.com> 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 `% 4096u` makes the wrap-around logic
> explicit and easier to understand. It clearly signals that the
> sequence number cycles through a range of 4096 values.
> It also makes the code robust against potential changes of the 4096
> upper limit, especially when it becomes a non power of 2 value while
> the AND(&) works solely for power of 2 values.
>
> The use of `% 4096u` also guarantees that the modulo operation is
> performed with unsigned arithmetic, preventing potential issues with
> the signed types.

...

> -                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (tx_seq+1)&0xfff;
> +                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (tx_seq+1)%4096u;

> -                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (pattrib->seqnum+1)&0xfff;
> +                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (pattrib->seqnum+1)%4096u;

To reduce churn, it would be nice to have another patch _before_ this
one that makes above clearer (by adding spaces) as we already new in
practice that it's hard to read:
So, this will become two lines in each of the cases, and in this patch
only one of those line will be touched.

...

Besides that I haven't found in the changelog if you have looked at
the entire driver code and checked that all usages of this field is
done in the same / similar way and no need to convert anything more
that these.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v5] staging: rtl8723bs: Use % 4096u instead of & 0xfff

[Samuel Abraham]

On Mon, Apr 7, 2025 at 5:17 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Mon, Apr 7, 2025 at 6:54 PM Abraham Samuel Adekunle
> <abrahamadekunle50@gmail.com> 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 `% 4096u` makes the wrap-around logic
> > explicit and easier to understand. It clearly signals that the
> > sequence number cycles through a range of 4096 values.
> > It also makes the code robust against potential changes of the 4096
> > upper limit, especially when it becomes a non power of 2 value while
> > the AND(&) works solely for power of 2 values.
> >
> > The use of `% 4096u` also guarantees that the modulo operation is
> > performed with unsigned arithmetic, preventing potential issues with
> > the signed types.
>
> ...
>
> > -                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (tx_seq+1)&0xfff;
> > +                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (tx_seq+1)%4096u;
>
> > -                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (pattrib->seqnum+1)&0xfff;
> > +                                               psta->BA_starting_seqctrl[pattrib->priority & 0x0f] = (pattrib->seqnum+1)%4096u;
>
> To reduce churn, it would be nice to have another patch _before_ this
> one that makes above clearer (by adding spaces) as we already new in
> practice that it's hard to read:
> So, this will become two lines in each of the cases, and in this patch
> only one of those line will be touched.

Okay, I will send a patchset for the change. The first one will add
the spaces, then the
second one will add the logic, and they can be applied in sequence.

>
> ...
>
> Besides that I haven't found in the changelog if you have looked at
> the entire driver code and checked that all usages of this field is
> done in the same / similar way and no need to convert anything more
> that these.

No, I have not checked other drivers. I will write a semantic patch to
match and transform such cases

Thanks
Adekunle.

Re: [PATCH v5] staging: rtl8723bs: Use % 4096u instead of & 0xfff

[Samuel Abraham]

On Mon, Apr 7, 2025 at 5:17 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Mon, Apr 7, 2025 at 6:54 PM Abraham Samuel Adekunle
> <abrahamadekunle50@gmail.com> 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 `% 4096u` makes the wrap-around logic
> > explicit and easier to understand. It clearly signals that the
> > sequence number cycles through a range of 4096 values.
> > It also makes the code robust against potential changes of the 4096
> > upper limit, especially when it becomes a non power of 2 value while
> > the AND(&) works solely for power of 2 values.
> >
> ...
>
> Besides that I haven't found in the changelog if you have looked at
> the entire driver code and checked that all usages of this field is
> done in the same / similar way and no need to convert anything more
> that these.

I have found more cases using a small Coccinelle semantic patch.
There were also cases in the media driver, but I did not touch those since it is
not allowed for an outreachy applicant.
I will send a patchset soon.

Thanks
Adekunle