Re: [PATCH v2] app/testpmd: fix lcore ID restriction

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 14 Jun 2024 10:01:02 +0100
Ferruh Yigit <ferruh.yigit@amd.com> wrote:

> On 6/13/2024 8:13 PM, Stephen Hemminger wrote:
> > On Thu, 13 Jun 2024 17:51:14 +0100
> > Ferruh Yigit <ferruh.yigit@amd.com> wrote:
> >   
> >>> Hi Sivaprasad,
> >>>
> >>> Is this '(lcoreid_t)' cast required? Because of integer promotion I
> >>> think result will be correct without casting.
> >>>
> >>> (And without integer promotion considered, casting needs to be done on
> >>> one of the variables, not to the result, because result may be already
> >>> cast down I think. Anyway this is not required for this case since
> >>> variables are u16.)
> >>>    
> >>
> >> Why casing required (for record) is,
> >> 'nb_txq' -> uint16_t, promoted to 'int'
> >> 'nb_fwd_ports' -> uint16_t, promoted to 'int'
> >> (nb_txq * nb_fwd_ports) -> result 'int'
> >> nb_fwd_lcores  -> 'uint32_t'
> >>
> >> comparison between 'int' & 'uint32_t' gives warning. After some compiler
> >> version it is smart enough to not give a warning, but casting is
> >> required for old compilers.
> >>
> >> And back to my comment above, casting one of the parameter to
> >> 'lcoreid_t' also works, as it forcing promotion to 'unsigned int'
> >> instead. But logically casting looks odd, so keeping casting result.  
> > 
> > Where is the integer promotion happening?
> >  
> 
> Raslan reported following compile error, this version of the patch has
> the cast, but merged version, v3, doesn't.
> 
> 
> ```
> ../../root/dpdk/app/test-pmd/config.c: In function 'icmp_echo_config_setup':
> ../../root/dpdk/app/test-pmd/config.c:5159:30: error: comparison between
> signed and unsigned integer expressions [-Werror=sign-compare]
>   if ((nb_txq * nb_fwd_ports) < nb_fwd_lcores)

That does look like a compiler bug. uint16 multiplied by uint16 should be uint16.
Not sure why DPDK keeps using small types like uint16 so much, doesn't have any
real benefit since all this is in registers.                             ^