Re: [PATCH v2 1/2] tracing/events: make __string() more general
From: Li Zefan
Date: Tue May 26 2009 - 21:58:08 EST
Steven Rostedt wrote:
> 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:
>> __string( cmd, rq->cmd_len * 3 )
>> construct_str( cmd )
> What does the construct_str do?
Ah, it's pseudo code.. Should be construct_str(__entry->cmd, rq),
this helper function convert rq->cmd (unsigned char) to a string of
No source string is available, so I'm not calling assign_str() in
>> 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.
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