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

From: Li Zefan
Date: Tue May 26 2009 - 21:23:28 EST

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 )

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. :(
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