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Subject: Re: [PATCH] blktrace: swap arg name "from" and "to" of 
	blk_add_trace_remap
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: "Alan D. Brunelle" <Alan.Brunelle@hp.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Li Zefan <lizf@cn.fujitsu.com>,
       Ingo Molnar <mingo@elte.hu>, Steven Rostedt <srostedt@redhat.com>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Jens Axboe <jens.axboe@oracle.com>,
       Arnaldo Carvalho de Melo <acme@redhat.com>
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> I believe this may be more messed up than you believe: Consider the last
> function: blk_log_remap. Notice how you are printing the /device/ and
> /sector_from/ as one pair. Isn't that confusing too: wouldn't one expect
> /device_from/ and /sector_from/ to be paired up?
>
> I ran into this problem a long time ago - hence the current code in
> blkparse contains:
>
>        if (act[0] == 'A') {    /* Remap */
>                get_pdu_remap(t, &r);
>                t->device = r.device_from;
>        }
>
> Which puts the device_from out at the beginning of the line, and then
> the 'device' + 'sector' (== 'sector_from') out at the end of the line.
> (Which, of course, is all confusing.)

Good viewpoint!
Currently, 99% blktrace user use blktrace and blkparse command, not
ftrace framework.
So, ABI changing is proper way?

I'd like to hear Jens and Li opinion.



> To really fix this, I think one needs to:
>
> (a) adjust the kernel to make "r.device_from" and "r.sector_from" refer
> to the same part of the trace, and the "r.device" and "t->sector" refer
> to the other part of the trace.
>
> (b) fix blkparse (& btt & ...) to do the same thing - not make
> adjustments for the stupid naming...
>
> If we leave the kernel->user space ABI alone:
>
> first u32: device mapped /from/
> second u32: device mapped /to/
> u64: sector mapped from
>
> We can then change the names. I've followed this e-mail up with a pair
> of kernel-side patches and will submit user side fixes for blkptrace later.