Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

Al Viro
2017-12-12T06:20:08Z

On Mon, Dec 11, 2017 at 08:02:24PM -0800, Jakub Kicinski wrote:
> On Mon, 11 Dec 2017 15:54:22 +0000, Al Viro wrote:
> > Essentially, it gives helpers for work with bitfields in fixed-endian.
> > Suppose we have e.g. a little-endian 32bit value with fixed layout;
> > expressing that as a bitfield would go like
> > struct foo {
> > unsigned foo:4; /* bits 0..3 */
> > unsigned :2;
> > unsigned bar:12; /* bits 6..17 */
> > unsigned baz:14; /* bits 18..31 */
> > }
> > Even for host-endian it doesn't work all that well - you end up with
> > ifdefs in structure definition and generated code stinks. For fixed-endian
> > it gets really painful, and people tend to use explicit shift-and-mask
> > kind of macros for accessing the fields (and often enough get the
> > endianness conversions wrong, at that). With these primitives
> >
> > struct foo v <=> __le32 v
> > v.foo = i ? 1 : 2 <=> v = le32_replace_bits(v, i ? 1 : 2, 0, 4)
> > f(4 + v.baz) <=> f(4 + le32_get_bits(v, 18, 14))
>
> Looks very useful. The [start bit, size] pair may not land itself
> too nicely to creating defines, though. Which is why in
> include/linux/bitfield.h we tried to use a shifted mask and work
> backwards from that single value what the start and size are. commit
> 3e9b3112ec74 ("add basic register-field manipulation macros") has the
> description. Could a similar trick perhaps be applicable here?

Umm... What's wrong with

#define FIELD_FOO 0,4
#define FIELD_BAR 6,12
#define FIELD_BAZ 18,14

A macro can bloody well expand to any sequence of tokens - le32_get_bits(v, FIELD_BAZ)
will become le32_get_bits(v, 18, 14) just fine. What's the problem with that?