[dpdk-dev] [RFC PATCH] eal:Add new API for parsing args at rte_eal_init time

[Chilikin, Andrey]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Neil Horman
> Sent: Thursday, June 4, 2015 2:56 PM
> To: Wiles, Keith
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [RFC PATCH] eal:Add new API for parsing args at
> rte_eal_init time
> 
> 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 at networkplumber.org>
> wrote:
> >
> > >On Wed,  3 Jun 2015 13:49:53 -0500
> > >Keith Wiles <keith.wiles at 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.

Before/after would be very useful, a lot of applications use only "-c" and "-n" EAL command line parameters and "-c" in many cases is redundant as application can calculate core mask from its own parameters, and "-n" just a required parameter which can be defaulted to a platform specific value. So in addition to rte_set_application_usage_hook() it would be nice to have some more general way of overwriting eal initialization parameters. 

> 
> Neil