linux-kernel - Re: [PATCH] staging: rtl8723bs: Replace `& 0xfff` with `% 4096u`

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Message-ID: <2025041402-colonize-rocker-1db1@gregkh>
Date: Mon, 14 Apr 2025 09:21:28 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Abraham Samuel Adekunle <abrahamadekunle50@...il.com>
Cc: outreachy@...ts.linux.dev, julia.lawall@...ia.fr,
	linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
	david.laight.linux@...il.com, dan.carpenter@...aro.org,
	andy@...nel.org
Subject: Re: [PATCH] staging: rtl8723bs: Replace `& 0xfff` with `% 4096u`

On Wed, Apr 09, 2025 at 06:48:02PM +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 `% 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.
> 
> Found by Coccinelle.
> 
> Suggested by Andy Shevchenko <andy@...nel.org>
> Signed-off-by: Abraham Samuel Adekunle <abrahamadekunle50@...il.com>
> ---
> Coccinelle semantic patch used to find cases:
> @@
> expression e;
> 
> @@
> * e & 0xfff
> 
> 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.

This is version 11, right?  What happened to the list of previous
versions and what changed down here?

confused,

greg k-h