Re: [PATCH] RDMA/hns: Work around broken constant propogation in gcc 8

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 16, 2021 at 12:05:28PM -0300, Jason Gunthorpe wrote:
> gcc 8.3 and 5.4 throw this:
> 
> In function 'modify_qp_init_to_rtr',
> ././include/linux/compiler_types.h:322:38: error: call to '__compiletime_assert_1859' declared with attribute error: FIELD_PREP: value too large for the field
>   _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> [..]
> drivers/infiniband/hw/hns/hns_roce_common.h:91:52: note: in expansion of macro 'FIELD_PREP'
>    *((__le32 *)ptr + (field_h) / 32) |= cpu_to_le32(FIELD_PREP(   \
>                                                     ^~~~~~~~~~
> drivers/infiniband/hw/hns/hns_roce_common.h:95:39: note: in expansion of macro '_hr_reg_write'
>  #define hr_reg_write(ptr, field, val) _hr_reg_write(ptr, field, val)
>                                        ^~~~~~~~~~~~~
> drivers/infiniband/hw/hns/hns_roce_hw_v2.c:4412:2: note: in expansion of macro 'hr_reg_write'
>   hr_reg_write(context, QPC_LP_PKTN_INI, lp_pktn_ini);
> 
> Because gcc has miscalculated the constantness of lp_pktn_ini:
> 
> 	mtu = ib_mtu_enum_to_int(ib_mtu);
> 	if (WARN_ON(mtu < 0)) [..]
> 	lp_pktn_ini = ilog2(MAX_LP_MSG_LEN / mtu);
> 
> Since mtu is limited to {256,512,1024,2048,4096} lp_pktn_ini is between 4
> and 8 which is compatible with the 4 bit field in the FIELD_PREP.
> 
> Work around this broken compiler by adding a 'can never be true'
> constraint on lp_pktn_ini's value which clears out the problem.
> 
> Fixes: f0cb411aad23 ("RDMA/hns: Use new interface to modify QP context")
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/infiniband/hw/hns/hns_roce_hw_v2.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)

Applied to for-rc

Jason