[PATCH v3 0/6] Add FIELD_MODIFY() helper

[Yury Norov]

On Thu, Apr 17, 2025 at 06:45:12PM +0100, Marc Zyngier wrote:
> On Thu, 17 Apr 2025 18:22:29 +0100,
> Andrew Lunn <andrew at lunn.ch> wrote:
> > 
> > On Thu, Apr 17, 2025 at 12:10:54PM +0100, Marc Zyngier wrote:
> > > On Thu, 17 Apr 2025 11:47:07 +0100,
> > > Luo Jie <quic_luoj at quicinc.com> wrote:
> > > > 
> > > > Add the helper FIELD_MODIFY() to the FIELD_XXX family of bitfield
> > > > macros. It is functionally similar as xxx_replace_bits(), but adds
> > > > the compile time checking to catch incorrect parameter type errors.
> > > > 
> > > > This series also converts the four instances of opencoded FIELD_MODIFY()
> > > > that are found in the core kernel files, to instead use the new
> > > > FIELD_MODIFY() macro. This is achieved with Coccinelle, by adding
> > > > the script field_modify.cocci.
> > > > 
> > > > The changes are validated on IPQ9574 SoC which uses ARM64 architecture.
> > > 
> > > We already have the *_replace_bits() functions (see
> > > include/linux/bitfield.h).
> > > 
> > > Why do we need extra helpers?
> > 
> > If you look at bitfield.h, the *_replace_bits() seem to be
> > undocumented internal macro magic, not something you are expected to
> > use. What you are expected to use in that file is however well
> > documented. The macro magic also means that cross referencing tools
> > don't find them.
> 
> $ git grep _replace_bits|  wc -l
> 1514

FIELD_PREP() only is used 10 times more.
 
> I think a bunch of people have found them, tooling notwithstanding.
> 
> As for the documentation, the commit message in 00b0c9b82663ac would
> be advantageously promoted to full-fledged kernel-doc.

The FIELD_MODIFY() and uxx_replace_bits() are simply different things.

FIELD_MODIFY() employs __BF_FIELD_CHECK(), which allows strict
parameters checking at compile time. And people like it. See
recent fixed-size GENMASK() series:

https://patchwork.kernel.org/comment/26283604/

The _replace_bits() functions return fixed-width values, and intended
for: "manipulating bitfields both in host- and fixed-endian", as the
very first line in the commit message says.

Those using _replace_bits() for something else abuse the API, and
should switch to FIELD_MODIFY().