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

[Al Viro]

On Tue, Dec 12, 2017 at 05:04:37PM -0800, Jakub Kicinski wrote:
> 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

Why not simply make nfp_eth_set_bit_config() static inline and be
done with that?  It's not that large and there are only few callers,
so...

> 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.

*shrug*

That thing would expand into "reg &= 15" in one case and "reg >>= 8; reg &= 15"
in another.  Either of which is cheaper than a function call, and definitely
cheaper than any kind of dynamic calculation of shift, no matter how implemented.