Re: [PATCH v2 1/2] tracing/events: make __string() more general

[Steven Rostedt]

On Wed, 27 May 2009, Li Zefan wrote:

> Steven Rostedt wrote:
> > 
> > On Tue, 26 May 2009, Li Zefan wrote:
> > 
> >> This changes __string(..., src) to __string(..., len), thus makes it more
> >> general, and then block TRACE_EVENT() can use it.
> >>
> >> Also introduce __fetch_str(), which is used in TP_assign() and returns
> >> the address of the string, while __get_str() is used in TP_print().
> >>
> >> [ Impact: TRACE_EVENT api change ]
> > 
> > What's the reason for the change?
> > 
> > Is it to handle NULL pointers? If so, lets change the ftrace.h to handle
> > it instead.
> > 
> 
> No, not for this purpose.
> 
> With __string(..., src), we need to have a source string, but sometimes no
> source string is available, and what we know is how big the string will be.
> 
> Take block trace events for example, I want to convert an unsigned char array
> into a string, after this patch, this can be done this way:
> 
> TP_STRUCT_entry(
> 	__string( cmd, rq->cmd_len * 3 )
> )
> 
> TP_fast_assign(
> 	construct_str( cmd )

What does the construct_str do?


> )
> 
> TP_prink("%s\n", get_str(cmd))
> 
> Seems Christoph also runs into this problem while doing his XFS tracing.
> (and NULL str ptr that I'm not suffering)
> 
> > This is why I like the Impact line really be a "rational" line.
> >  "TRACE_EVENT api change" is meaningless to me.
> > 
> 
> So what should this impact be. :(

That's the point. I don't know the purpose of the patch except that it 
changes the API. But changing the API is the means to the end. I'm still 
confused by what the end is.

-- Steve

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