linux-kernel - Re: [PATCH 01/20] bitfield: introduce HWORD

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Message-ID: <aEw7LBpmkfOqZgf1@yury>
Date: Fri, 13 Jun 2025 10:52:28 -0400
From: Yury Norov <yury.norov@...il.com>
To: Robin Murphy <robin.murphy@....com>
Cc: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
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Subject: Re: [PATCH 01/20] bitfield: introduce HWORD_UPDATE bitfield macros

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()?