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

Jakub Kicinski
2017-12-13T01:04:48Z

On Wed, 13 Dec 2017 00:36:59 +0000, Al Viro wrote:
> On Tue, Dec 12, 2017 at 03:59:33PM -0800, Jakub Kicinski wrote:
> > > +static __always_inline __##type type##_replace_bits(__##type old, \
> > > + base val, base mask) \
> > > +{ \
> > > + __##type m = to(mask); \
> > > + if (__builtin_constant_p(val) && \
> >
> > Is the lack of a __builtin_constant_p(mask) test intentional? Sometimes
> > the bitfield is a packed array and people may have a helper to which
> > only the mask is passed as non-constant and the value is implied by the
> > helper, thus constant.
>
> If the mask in non-constant, we probably shouldn't be using that at all;
> could you show a real-world example where that would be the case?

FIELD_* macros explicitly forbid this, since the code would be...
suboptimal with the runtime ffsl. Real life examples are the hackish
macro NFP_ETH_SET_BIT_CONFIG() in

drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c

I remember there was also some Renesas code.. maybe this:

https://patchwork.kernel.org/patch/9881279/

it looks like cpg_z_clk_recalc_rate() and cpg_z2_clk_recalc_rate() only
differ in mask.