linux-kernel - Re: [PATCH] nfp: convert nfp_eth_set_bit

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Date:   Wed, 4 Oct 2017 19:06:21 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Manoj Gupta <manojgupta@...omium.org>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Joe Perches <joe@...ches.com>,
        "David S . Miller" <davem@...emloft.net>,
        Simon Horman <simon.horman@...ronome.com>,
        Dirk van der Merwe <dirk.vandermerwe@...ronome.com>,
        oss-drivers@...ronome.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Renato Golin <renato.golin@...aro.org>,
        Guenter Roeck <groeck@...omium.org>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH] nfp: convert nfp_eth_set_bit_config() into a macro

On Wed, 4 Oct 2017 18:50:04 -0700, Manoj Gupta wrote:
> On Wed, Oct 4, 2017 at 5:56 PM, Jakub Kicinski wrote:
> > On Wed, 4 Oct 2017 17:38:22 -0700, Manoj Gupta wrote:  
> >> On Wed, Oct 4, 2017 at 4:25 PM, Jakub Kicinski wrote:  
> >> > On Wed, 4 Oct 2017 16:16:49 -0700, Matthias Kaehlcke wrote:  
> >> >> > > Thanks for the suggestion. This seems a viable alternative if David
> >> >> > > and the NFP owners can live without the extra checking provided by
> >> >> > > __BF_FIELD_CHECK.  
> >> >> >
> >> >> > The reason the __BF_FIELD_CHECK refuses to compile non-constant masks
> >> >> > is that it will require runtime ffs on the mask, which is potentially
> >> >> > costly.  I would also feel quite stupid adding those macros to the nfp
> >> >> > driver, given that I specifically created the bitfield.h header to not
> >> >> > have to reimplement these in every driver I write/maintain.  
> >> >>
> >> >> That make sense, thanks for providing more context.
> >> >>  
> >> >> > Can you please test the patch I provided in the other reply?  
> >> >>
> >> >> With this patch there are no errors when building the kernel with
> >> >> clang.  
> >> >
> >> > Cool, thanks for checking!  I will run it through full tests and queue
> >> > for upstreaming :)  
> >>
> >> Just to let you know, using __BF_FIELD_CHECK macro will not Link with
> >> -O0 (GCC or Clang)  since references to __compiletime_assert_xxx will
> >> not be cleaned up.  
> >
> > Do you mean the current nfp_eth_set_bit_config() will not work with -O0
> > on either complier, or any use of __BF_FIELD_CHECK() will not compile
> > with -O0?  
> 
> Any use of __BF_FIELD_CHECK. The code will compile but not link since
> calls to ____compiletime_assert_xxx (added by compiletime_assert
> macro) will not be removed in -O0.

Why would that be, it's just a macro?  Does it by extension mean any
use of BUILD_BUG_ON_MSG() will not compile with -O0?