linux-kernel - Re: [RFC][PATCH] new byteorder primitives

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Date:   Tue, 12 Dec 2017 17:04:37 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] new byteorder primitives -
 ..._{replace,get}_bits()

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.