Re: [dpdk-dev] [PATCH] ixgbe: fix build with gcc 4.4

[Vlad Zolotarov <vladz@cloudius-systems.com>]

On 04/14/15 17:53, Thomas Monjalon wrote:
> 2015-04-14 17:30, Vlad Zolotarov:
>> On 04/14/15 17:17, Thomas Monjalon wrote:
>>> 2015-04-14 16:38, Vlad Zolotarov:
>>>> On 04/14/15 16:06, Ananyev, Konstantin wrote:
>>>>> From: Vlad Zolotarov [mailto:vladz@cloudius-systems.com]
>>>>>> On 04/14/15 12:31, Thomas Monjalon wrote:
>>>>>>> -	struct rte_eth_dev_info dev_info = { 0 };
>>>>>>> +	struct rte_eth_dev_info dev_info = { .max_rx_queues = 0 };
>>>>>> Hmmm... Unless I miss something this and one above would zero only a
>>>>>> single field - "max_rx_queues"; and would leave the rest uninitialized.
>>>>>> The original code intend to zero the whole struct. The alternative to
>>>>>> the original lines could be usage of memset().
>>>>> As I understand, in that case compiler had to set all non-explicitly initialised members to 0.
>>>>> So I think we are ok here.
>>>> Yeah, I guess it does zero-initializes the rest
>>>> (https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html) however I
>>>> don't understand how the above change fixes the error if it complains
>>>> about the dev_info.driver_name?
>>> As only 1 field is required, I chose the one which should not be removed
>>> from this structure in the future.
>> I don't follow - where/why only one field is required? The function u
>> are patching uses "rx_offload_capa" field. Or u mean this gcc version
>> requires only one field? If so, could u, please, provide the errata u
>> are referring, since standard doesn't require any field and {0} is an
>> absolutely legal (and proper) initializer in this case...
> Honestly I don't really care what is "legal". The most important is to make
> it working with most C compilers with minimal overhead.

It's not just a "legal" - it's the most correct and robust way of 
initializing the struct that is promised to always work correctly. See 
here 
http://stackoverflow.com/questions/11152160/initializing-a-struct-to-0. 
What u hit here is (as appears) a well known Bug #53119 in gcc (see here 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119).

Have u considered adding the compilation options like 
-Wno-missing-braces that would silence this warning for say gcc versions 
below 4.7?

> You're right about the variable choice: rx_offload_capa is more appropriate.
> Are you OK for a v2 replacing max_rx_queues by rx_offload_capa?
>
>>>> What I'm trying to say - the proposed fix is completely unclear and
>>>> confusing. Think of somebody reading this line in a month from today -
>>>> he wouldn't get a clue why is it there, why to explicitly set
>>>> max_rx_queues to zero and leave the rest be zeroed automatically... Why
>>>> to add such artifacts to the code instead of just zeroing the struct
>>>> with a memset() and putting a good clear comment above it explaining why
>>>> we use a memset() and not and initializer?
>>> We can make it longer yes.
>>> I think you agree we should avoid extra lines if not needed.
>>> In this case, when reading "= { .field = 0 }", it seems clear our goal
>>> is to zero the structure (it is to me).
>>> I thought it is a basic C practice.
>>>
>>> You should try "git grep '\.[^ ]\+ *= *0 *}'" to be convinced that we are
>>> not going to comment each occurence of this coding style.
>>> But it must be explained in the coding style document. Agree?
>