Re: [PATCH] nfp: convert nfp_eth_set_bit_config() into a macro

[Manoj Gupta]

On Wed, Oct 4, 2017 at 7:06 PM, Jakub Kicinski
<jakub.kicinski@xxxxxxxxxxxxx> wrote:
> 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?

You have to look at the the code added once the macro is expanded :).
Please look at implementation of compiletime_assert at
http://elixir.free-electrons.com/linux/v4.12.14/source/include/linux/compiler.h#L507
It creates a call to __compiler_assert_xxx inside a loop which is not
cleaned up in -O0.

Thanks,
Manoj