Re: [PATCH 1/9] bitfield: add FIELD_GET_SIGNED()

[David Laight <david.laight.linux@gmail.com>]

On Mon, 20 Apr 2026 10:43:08 +0200
Johannes Berg <johannes@sipsolutions.net> wrote:

...
> I (personally) tend to prefer the "__MAKE_OP" versions (*_get_bits()
> etc.), in particular because WiFi and firmware interfaces deal a lot
> with fixed endian fields.
> 
> Any chance it'd be simple to generate u32_get_bits_signed() etc.? Could
> be especially useful for le32_get_bits_signed() for example, to have the
> endian conversion built-in unlike FIELD_GET_SIGNED().

The problem is that the number of options explodes.
You need PREP/GET/GET_SIGNED functions x_64() x_64le() x_64be() x_32() x_32le()
x_32be() x_16le() and x_16be().
I make that 24 functions.
For UPDATE you also need x_16() and x_8() for another 10.
So at least 34 definitions - it is all getting silly.
(And I've excluded the pointless 8/16 bit functions where the result
is promoted to signed int.)

Then you have the problem that some (common) architectures don't have
byteswap instructions, but do have byteswapping memory accesses (maybe not
16bit). Which means that is it generally better to byteswap the value
before passing to FIELD_GET() and after building the full 'word' using
FIELD_PREP() (and just | for single bits).
(Except it is probably always better to byteswap constants.)

It is also worth nothing that FIELD_PREP() will reject requests to
fill signed fields with negative constants.

	David

> 
> johannes
>