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Date: Wed, 27 May 2009 10:33:47 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@elte.hu>, Jens Axboe <jens.axboe@oracle.com>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Tom Zanussi <tzanussi@gmail.com>, "Theodore Ts'o" <tytso@mit.edu>,
       Steven Whitehouse <swhiteho@redhat.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Jeff Moyer <jmoyer@redhat.com>, Christoph Hellwig <hch@infradead.org>,
       FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] tracing/events: make __string() more general
References: <4A1B6A2F.2040703@cn.fujitsu.com> <4A1B6A4C.3070407@cn.fujitsu.com> <alpine.DEB.2.00.0905261655120.26705@gandalf.stny.rr.com> <4A1C9645.7010107@cn.fujitsu.com> <alpine.DEB.2.00.0905262147470.27902@gandalf.stny.rr.com> <4A1C9E70.8000609@cn.fujitsu.com> <alpine.DEB.2.00.0905262202390.27902@gandalf.stny.rr.com>
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>> Ah, it's pseudo code.. Should be construct_str(__entry->cmd, rq),
>> this helper function convert rq->cmd (unsigned char[]) to a string of
>> hex dump.
>>
>> No source string is available, so I'm not calling assign_str() in
>> TP_fast_assign.
> 
> OK, I think I have a better understanding of what you are doing.
> 
> But, I don't like changing the interface for string by adding that 
> "strlen" to all users. I find that very ugly.
> 

Yeah, it does look somewhat ugly..

> Could we instead perhaps add a __dynamic_array? That would take a length, 
> and append itself to after the strings?
> 

I thought about this actually. ;)

What troubled me is to give this new macro a better name.

We have __array() which is fixed-size, but __dynamic_array() is not
a dynamic-size version of __array(). How about __dyn_string() or
__dynamic_string()?

> stage 1:
> 
> #define __dynamic_array(item, len)	int __dyn_loc_##item;
> 
> stage 2:
> 
> #define __dynamic_array(item, len)	int item;
> 
> stage 3:
> 
> #define __get_dynamic_array(item)	\
> 	((void *)__entry + __entry->__str_loc_##field
> 
> stage 4:
> 
> #define __dynamic_array(item, len)	\
> 	__str_offsets.item = __str_size + \
> 		offsetof(typeof(*entry), __str_data);	\
> 	__str_size += len;
> 

Then we can make __string() reuse __dynamic_array().

> 
> And so on. You should be able to do something like this, and make it more 
> generic.
>