netdev - Re: [PATCH] nfp: convert nfp_eth_set_bit

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Message-ID: <20171004231649.GP173745@google.com>
Date:   Wed, 4 Oct 2017 16:16:49 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     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>,
        Manoj Gupta <manojgupta@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH] nfp: convert nfp_eth_set_bit_config() into a macro

Hi Jakub,

El Wed, Oct 04, 2017 at 03:22:03PM -0700 Jakub Kicinski ha dit:

> On Wed, 4 Oct 2017 11:49:57 -0700, Matthias Kaehlcke wrote:
> > Hi Joe,
> > 
> > El Wed, Oct 04, 2017 at 11:07:19AM -0700 Joe Perches ha dit:
> > 
> > > On Tue, 2017-10-03 at 13:05 -0700, Matthias Kaehlcke wrote:  
> > > > nfp_eth_set_bit_config() is marked as __always_inline to allow gcc to
> > > > identify the 'mask' parameter as known to be constant at compile time,
> > > > which is required to use the FIELD_GET() macro.
> > > > 
> > > > The forced inlining does the trick for gcc, but for kernel builds with
> > > > clang it results in undefined symbols:  
> > > 
> > > Can't you use local different FIELD_PREP/FIELD_GET macros
> > > with a different name without the BUILD_BUG tests?
> > > 
> > > i.e.:
> > > 
> > > #define NFP_FIELD_PREP(_mask, _val)				\
> > > ({								\
> > > 	((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask);	\
> > > })
> > > 
> > > #define NFP_FIELD_GET(_mask, _reg)				\
> > > ({								\
> > > 	(typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask));	\
> > > })
> > > 
> > > Then the __always_inline can be removed from
> > > nfp_eth_set_bit_config too.  
> > 
> > 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.

Thanks!

Matthias