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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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.
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?
>