Re: [PATCH 01/20] bitfield: introduce HWORD_UPDATE bitfield macros

Nicolas Frattaroli
2025-06-16T12:29:37Z

Hello,

On Friday, 13 June 2025 16:52:28 Central European Summer Time Yury Norov wrote:
> On Fri, Jun 13, 2025 at 02:54:50PM +0100, Robin Murphy wrote:
> > On 2025-06-12 7:56 pm, Nicolas Frattaroli wrote:
> > > Hardware of various vendors, but very notably Rockchip, often uses
> > > 32-bit registers where the upper 16-bit half of the register is a
> > > write-enable mask for the lower half.
> > >
> > > This type of hardware setup allows for more granular concurrent register
> > > write access.
> > >
> > > Over the years, many drivers have hand-rolled their own version of this
> > > macro, usually without any checks, often called something like
> > > HIWORD_UPDATE or FIELD_PREP_HIWORD, commonly with slightly different
> > > semantics between them.
> > >
> > > Clearly there is a demand for such a macro, and thus the demand should
> > > be satisfied in a common header file.
> > >
> > > Add two macros: HWORD_UPDATE, and HWORD_UPDATE_CONST. The latter is a
> > > version that can be used in initializers, like FIELD_PREP_CONST. The
> > > macro names are chosen to not clash with any potential other macros that
> > > drivers may already have implemented themselves, while retaining a
> > > familiar name.
> >
> > Nit: while from one angle it indeed looks similar, from another it's even
> > more opaque and less meaningful than what we have already. Personally I
> > cannot help but see "hword" as "halfword", so logically if we want 32+32-bit
> > or 8+8-bit variants in future those would be WORD_UPDATE() and
> > BYTE_UPDATE(), right? ;)
> >
> > It's also confounded by "update" not actually having any obvious meaning at
> > this level without all the implicit usage context. FWIW my suggestion would
> > be FIELD_PREP_WM_U16, such that the reader instantly sees "FIELD_PREP with
> > some additional semantics", even if they then need to glance at the
> > kerneldoc for clarification that WM stands for writemask (or maybe WE for
> > write-enable if people prefer). Plus it then leaves room to easily support
> > different sizes (and potentially even bonkers upside-down Ux_WM variants?!)
> > without any bother if we need to.
>
> I like the idea. Maybe even shorter: FIELD_PREP_WM16()?
>

I do think FIELD_PREP_WM16() is a good name. If everyone is okay with this
as a name, I will use it in v2 of the series. And by "everyone" I really
mean everyone should get their hot takes in before the end of the week,
as I intend to send out a v2 on either Friday or the start of next week
to keep the ball rolling, but I don't want to reroll a 20 patch series
with a trillion recipients more than is absolutely necessary.

To that end, I'd also like to get some other naming choices clarified.

As I gathered, these two macros should best be placed in its own header.
Is include/linux/hw_bitfield.h a cromulent choice, or should we go with
include/linux/hw_bits.h?

Furthermore, should it be FIELD_PREP_WM16_CONST or FIELD_PREP_CONST_WM16?
I'm personally partial to the former.

And finally, is it okay if I leave out refactoring Intel's
_MASKED_FIELD() or should I see if I can at least replace its
implementation while I'm at it?

For less opinionated changes, I'll also change all the `U` literal
suffixes to `UL` wherever I've added them. As I understand it, it doesn't
really make a difference in these instances, but `UL` is more prevalent
in the kernel.

Kind regards,
Nicolas Frattaroli