Re: [RFC PATCH] eal:Add new API for parsing args at rte_eal_init time

[Thomas F Herbert <therbert@redhat.com>]

On 6/4/15 9:55 AM, Neil Horman wrote:
> On Thu, Jun 04, 2015 at 11:50:33AM +0000, Wiles, Keith wrote:
>> Hi Stephen
>>
>> On 6/3/15, 7:12 PM, "Stephen Hemminger" <stephen@networkplumber.org> wrote:
>>
>>> On Wed,  3 Jun 2015 13:49:53 -0500
>>> Keith Wiles <keith.wiles@intel.com> wrote:
>>>
>>>> +/* Launch threads, called at application init() and parse app args. */
>>>> +int
>>>> +rte_eal_init_parse(int argc, char **argv,
>>>> +		int (*parse)(int, char **))
>>>> +{
>>>> +	int	ret;
>>>> +
>>>> +	ret = rte_eal_init(argc, argv);
>>>> +	if ((ret >= 0) && (parse != NULL)) {
>>>> +		argc -= ret;
>>>> +		argv += ret;
>>>
>>> This won't work C is call by value.
>>
>> I tested this routine with Pktgen (again), which has a number of
>> application options and it appears to work correctly. Can you explain why
>> this will not work?
>>
>> Regards,
>> ++Keith
>>>
>>
>>
>
> Stephen was thinking that your intent was to have argc, and argv modified at the
> call site of this function (i.e. if you called rte_eal_init_parse from main(),
> then after the call to rte_ela_init_parse, argc would be reduced by ret and argv
> would point forward in memory ret bytes in the main function, but they wont.  It
> doesn't matter though, because you return ret, so the caller can do that
> movement themselves.  As you note, it works.
>
> Note that if it was your intention to have argc and argv modified at the call
> site, then Stephen is right and this is broken, you need to modify the prototype
> to be:
> int rte_eal_init_parse(int *argc, char ***argv)
>
> and do a dereference when modifying the parameters so the change is seen at the
> call site.
>
> That said, I'm not sure theres much value in adding this to the API.  For one,
> it implies that dpdk arguments need to come first on the command line.  While
> all the example applications do that, theres no requirement that they do so, and
> this function silently implies that they have to, so any existing applications
> in the wild that violate that assumption are enjoined from using this
>
> It also doesn't really save any code.  If we pick an example app (I'll us
> l2fwd-jobstats), We currently have this:
>
> 	/* init EAL */
>          ret = rte_eal_init(argc, argv);
>          if (ret < 0)
>                  rte_exit(EXIT_FAILURE, "Invalid EAL arguments\n");
>          argc -= ret;
>          argv += ret;
>
>          /* parse application arguments (after the EAL ones) */
>          ret = l2fwd_parse_args(argc, argv);
> 	if (ret < 0)
>                  rte_exit(EXIT_FAILURE, "Invalid L2FWD arguments\n");
>
> With your new API we would get this:
>
> 	ret = rte_eal_init_parse(argc, argv, l2fwd_parse_args)
>          if (ret < 0)
>                  rte_exit(EXIT_FAILURE, "Invalid arguments - not sure what\n");
>
> Its definately 5 fewer lines of source, but it doesn't save any execution
> instructions, and for the effort of that, you loose the ability to determine if
> it was a DPDK argument or an application argument that failed.
>
> Its not a bad addition, I'm just not sure its worth having to take on the
> additional API surface to include.  I'd be more supportive if you could enhance
> the function to allow the previously mentioned before/after flexibiilty.  Then
> we could just deprecate rte_eal_init as an API call entirely, and use this
> instead.
+1.

Also, I think rte_set_application_usage_hook() callback could be used by 
app writers for implementing usage() for a conventional  "<program> -h" 
like capability to print all usage including both eal and app specific 
args even if the eal args are not correct. This is an alternative to 
calling eal_init() first and bombing before printing all usage.

--TFH

> Neil
>