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

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 6/14/2024 4:27 PM, Stephen Hemminger wrote:
> 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.
>

Probably, this warning goes away with newer version of compiler.

> Not sure why DPDK keeps using small types like uint16 so much, doesn't have any
> real benefit since all this is in registers.                             ^