btt -B doesn't output anything with DM device

Hi, Guys,

I am trying to visualize io pattern by bno_plot.
bno_plot requires an output from btt -B but
btt -B doesn't output anything for DM devices (e.g. /dev/mapper/mydev) while it does for real block devices (e.g. /dev/sdc).

For narrowing down the problem
I reproduced this problem with simple linear device.

# Setup

root@Freedom:/home/akira# dmsetup create mydev --table "0 100000 linear /dev/sdc 0"
root@Freedom:/home/akira# dmsetup ls
pdc_fgaahfadb   (254:0)
mydev   (254:1) <- this

# Trace

root@Freedom:/home/akira# cat devlist
/dev/mapper/mydev
/dev/sdc
root@Freedom:/home/akira# blktrace -w 30 -I devlist -D mytrace
=== dm-1 ===
  CPU  0:                    0 events,        0 KiB data
  CPU  1:                  378 events,       18 KiB data
  Total:                   378 events (dropped 0),       18 KiB data
=== sdc ===
  CPU  0:                    0 events,        0 KiB data
  CPU  1:                  781 events,       37 KiB data
  Total:                   781 events (dropped 0),       37 KiB data
root@Freedom:/home/akira# ls mytrace/
dm-1.blktrace.0  dm-1.blktrace.1  sdc.blktrace.0  sdc.blktrace.1

# btt
root@Freedom:/home/akira# btt -i mytrace/sdc.blktrace.* -B sdc
root@Freedom:/home/akira# btt -i mytrace/dm-1.blktrace.* -B dm-1  
root@Freedom:/home/akira# ls *.dat
sdc_008,032_c.dat  sdc_008,032_r.dat

btt doesn't output anything with dm-1.blktrace.* . Is this intentionally designed or just a (known/unknown) bug? Or, do I forget something?

Thanks,
Akira