[dpdk-dev] [PATCH v3 2/4] ethdev: move error checking macros to header

[Adrien Mazarguil]

On Fri, Nov 06, 2015 at 05:22:27PM +0000, Bruce Richardson wrote:
> On Fri, Nov 06, 2015 at 05:10:07PM +0000, Bruce Richardson wrote:
> > On Thu, Nov 05, 2015 at 04:09:18PM +0100, Adrien Mazarguil wrote:
> > > 
> > > I won't argue against this as it's obviously more complex than the original
> > > method, however note that users of the RTE_PMD_DEBUG_TRACE() macro do not
> > > have to modify their code. They shouldn't care about the implementation.
> > > 
> > > Also note that we can do much cleaner code if we drop the all macros
> > > implementation using a (much easier to debug) static inline function,
> > > only perhaps with a wrapper macro that provides __LINE__, __func__ and
> > > __FILE__ as arguments. Nontrival code shouldn't be done in macros anyway.
> > > 
> > Getting something working with __FILE__ and probably __LINE__ would be easy enough
> > with a helper macro, but __func__ is not so easy as it's not a preprocessor symbol
> > [since the pre-processor has no idea what function you are in].
> > 
> > However, using func, here is the best I've come up with so far. It's not that
> > pretty, but it's probably easier to work with than the macro version.
> > 
> >  #ifdef RTE_LIBRTE_ETHDEV_DEBUG
> >  -#define RTE_PMD_DEBUG_TRACE(fmt, args...) \
> >  -               RTE_LOG(ERR, PMD, "%s: " fmt, __func__, ## args)
> >  +#define RTE_PMD_DEBUG_TRACE(...) \
> >  +               rte_pmd_debug_trace(__func__, __VA_ARGS__)
> >  +
> >  +static inline void
> >  +rte_pmd_debug_trace(const char *func_name, const char *fmt, ...)
> >  +{
> >  +       static __thread char buffer[128];
> >  +       char *out_buf = buffer;
> >  +       unsigned count;
> >  +       va_list ap;
> >  +
> >  +       count = snprintf(buffer, sizeof(buffer), "%s: %s", func_name, fmt);
> >  +       if (count >= sizeof(buffer)) { // truncated output
> >  +               char *new_buf = malloc(count + 1);
> >  +               if (new_buf == NULL) // no memory, just print 128 chars
> >  +                       goto print_buffer;
> >  +               snprintf(new_buf, count + 1, "%s: %s", func_name, fmt);
> >  +               va_start(ap, fmt);
> >  +               rte_vlog(RTE_LOG_ERR, RTE_LOGTYPE_PMD, buffer, ap);
> >  +               va_end(ap);
> >  +               free(new_buf);
> >  +               return;
> >  +       }
> >  +
> >  +print_buffer:
> >  +       va_start(ap, fmt);
> >  +       rte_vlog(RTE_LOG_ERR, RTE_LOGTYPE_PMD, out_buf, ap);
> >  +       va_end(ap);
> >  +}
> >   #else
> >   #define RTE_PMD_DEBUG_TRACE(fmt, args...)
> >   #endif
> > 
> > Comments or improvements?

Such a function shouldn't malloc() anything. The entire line should fit on
the stack (crashing is fine if it does not, then it's probably a bug). We
did something in two passes along these lines in mlx5_defs.h (not pretty but
quite useful):

 /* Allocate a buffer on the stack and fill it with a printf format string. */
 #define MKSTR(name, ...) \
         char name[snprintf(NULL, 0, __VA_ARGS__) + 1]; \
         \
         snprintf(name, sizeof(name), __VA_ARGS__)

Untested but I guess modifying that function accordingly would look like:

 static inline void
 rte_pmd_debug_trace(const char *func_name, const char *fmt, ...)
 {
         va_list ap;
         va_start(ap, fmt);

         static __thread char buffer[vsnprintf(NULL, 0, fmt, ap)];

         va_end(ap);
	 va_start(ap, fmt);
         vsnprintf(buffer, sizeof(buffer), fmt, ap);
	 va_end(ap);
         rte_log(RTE_LOG_ERR, RTE_LOGTYPE_PMD, "%s: %s", func_name, buffer);
 }

> And here's the version if we are happy to have file and line number instead of
> function name. I think this might be the best option.
> 
> /Bruce
> 
>  #ifdef RTE_LIBRTE_ETHDEV_DEBUG
>  -#define RTE_PMD_DEBUG_TRACE(fmt, args...) \
>  -               RTE_LOG(ERR, PMD, "%s: " fmt, __func__, ## args)
>  +#define RTE_PMD_DEBUG_TRACE(...) \
>  +       RTE_LOG(ERR, PMD, __FILE__", " RTE_STR(__LINE__) ": " __VA_ARGS__)
>   #else
>  -#define RTE_PMD_DEBUG_TRACE(fmt, args...)
>  +#define RTE_PMD_DEBUG_TRACE(...)
>   #endif

Much cleaner indeed, however __func__ might be useful when comparing log
outputs from different source code versions. I think we should keep it.

-- 
Adrien Mazarguil
6WIND